Table of Contents

reimburse brokers, banks and other nominees, fiduciaries and custodians for reasonable expenses incurred by them in forwarding proxy materials to beneficial owners of our common stock.

        No person is authorized to give any information or to make any representation not contained in this proxy statement, and, if it is given or made, you should not rely on that information or

Table of Contents

representation as having been authorized by us. The delivery of this proxy statement does not imply that the information herein has remained unchanged since the date of this proxy statement.

Purposes of the Annual Meeting

        The purposes of the Annual Meeting are to: (1) consider and vote upon the election of eight membersdirectors to the Board to serve until the next meeting of stockholders and until their successors are duly elected and qualify ("Proposal One"); (2) consider and vote upon the ratification of the appointment of KPMG LLP ("KPMG") as our independent registered public accounting firm for the fiscal year ending December 31, 20182019 ("Proposal Two"); (3) consider and vote upon the advisory vote to approve the compensation of our named executive officers ("Proposal Three"); and (4) transact such other business as may properly come before the Annual Meeting or any postponement or adjournment thereof. Our Board knows of no other matters to be brought before the Annual Meeting.

Stockholders Entitled to Vote

        The close of business on March 21, 2018,14, 2019, has been fixed as the record date (the "Record Date") for the determination of stockholders entitled to receive notice of and to vote at the Annual Meeting. Only stockholders of record as of the close of business on the Record Date or their duly authorized proxies are entitled to vote at the Annual Meeting. On the Record Date, our outstanding securities entitled to vote at the Annual Meeting consisted of 34,468,08336,681,134 shares of common stock. Each share of common stock is entitled to one vote. Votes may not be cumulated in the election of directors.

How to Vote

        If your shares are registered directly in your name with our transfer agent, American Stock Transfer & Trust Company, LLC, you are considered the stockholder of record with respect to those shares, and the Notice was sent directly to you by us. You may authorize your proxy via the Internet by following the instructions provided in the Notice. If you request printed copies of the proxy materials by mail, you may also authorize your proxy by filling out the proxy card included with the materials or by calling the toll-free number found on the proxy card.

        If your shares are held in an account at a brokerage firm, bank, broker-dealer, or other similar organization, then you are the beneficial owner of shares held in "street name," and the Notice was forwarded to you by that organization. The organization holding your account is considered the stockholder of record for purposes of voting at the Annual Meeting. As a beneficial owner, you have the right to instruct that organization on how to vote the shares held in your account. Those instructions are contained in a "vote instruction form." If you request printed copies of the proxy materials by mail, you will receive a vote instruction form. You should instruct your broker or nominee how to vote your shares by following the voting instructions on the voting instruction form provided by your broker or nominee.

        In addition, you may vote in person at the Annual Meeting as described below under the heading "Attending and Voting at the Annual Meeting."

Table of Contents

Attending and Voting at the Annual Meeting

        Only stockholders as of the Record Date, or their duly appointed proxies, may attend and vote at the Annual Meeting. Stockholders may be asked to present valid picture identification such as a driver's license or passport and proof of stock ownership as of the Record Date. If you are not a stockholder of record but hold shares through a broker or nominee (i.e., in street name), you should provide proof of beneficial ownership on the Record Date, such as your most recent account statement, a copy of the voting instruction card provided by your broker, trustee or nominee, or other similar evidence of ownership. The use of cell phones, smartphones, pagers, recording and photographic equipment and/or computers is not permitted in the meeting room at the Annual Meeting.

Table of Contents

        Shares held in your name as the stockholder of record may be voted in person at the Annual Meeting. If you are not a stockholder of record but hold shares through a broker or nominee (i.e., in street name), you may vote your shares in person only if you obtain a legal proxy from the broker, trustee or nominee that holds your shares authorizing you to vote the shares. Even if you plan to attend the Annual Meeting, we recommend that you also authorize your proxy or submit voting instructions prior to the meetingAnnual Meeting as described below so that your vote will be counted if you later decide not to attend the Annual Meeting.

Voting Without Attending the Annual Meeting

        Whether you hold shares directly as the stockholder of record or beneficially in street name, you may direct how your shares are voted without attending the Annual Meeting. If you are a stockholder of record, you may authorize your proxy to vote your shares, either by mail or via the Internet, or by calling the toll-free number found on the Notice and the proxy card.

        Proxies authorized properly via one of the methods discussed above will be voted in accordance with the instructions contained therein. If the proxy is authorized but voting directions are not made, the proxy will be voted "FOR" each of the eight director nominees, "FOR" the ratification of the appointment of KPMG as our independent registered public accounting firm for the fiscal year ending December 31, 2018,2019, "FOR" the advisory vote to approve the compensation of our named executive officers as described in this proxy statement, and in such manner as the proxy holders named on the proxy (the "Proxy Agents"), in their discretion, determine upon such other business as may properly come before the Annual Meeting or any postponement or adjournment thereof.

        If your shares of common stock are held through a broker, bank or other nominee (collectively referred to as "brokers"), under applicable rules of the New York Stock Exchange (the "NYSE") (the exchange on which our common stock is traded), the brokers will vote your shares according to the specific instructions they receive from you. If a broker that holds shares of our common stock for a beneficial owner does not receive voting instructions from that owner, the broker may vote on the proposal only if it is considered a "routine" matter under the NYSE's rules, such as this year's ratification of the appointment of KPMG as our independent registered public accounting firm for the fiscal year ending December 31, 20182019 (Proposal 2)Two), or leave that owner's shares unvoted. Pursuant to the rules of the NYSE, the election of directors (Proposal 1)One) and the advisory vote to approve the compensation of our named executive officers (Proposal 3)Three) are not "routine" matters as to which brokerage firms may vote in their discretion on behalf of clients who have not furnished voting instructions. A broker non-vote occurs when a bank, broker or other person holding shares for a beneficial owner does not vote on a particular proposal because that holder does not have discretionary voting power for that particular item and has not received voting instructions from the beneficial owner.

Table of Contents

        The proposals set forth in this proxy statement constitute the only business that the Board intends to present at the Annual Meeting. The proxy does, however, confer discretionary authority upon the Proxy Agents or their substitutes, to vote on any other business that may properly come before the Annual Meeting. If the Annual Meeting is postponed or adjourned, the Proxy Agents can vote your shares on the new meeting date as well, unless you have revoked your proxy.

Quorum

        Holders of a majority of our outstanding common stock entitled to vote must be present, in person or by proxy, at the Annual Meeting for a quorum to exist. If the shares present in person or by proxy at the Annual Meeting do not constitute a quorum, the Annual Meeting may be adjourned by the chairman of the Annual Meeting to a date not more than 120 days after the Record Date without notice other than announcement at the Annual Meeting. Shares that are voted "FOR," "AGAINST,"

Table of Contents

"ABSTAIN, "ABSTAIN," or, with respect to the election of directors, "WITHHOLD," or, with respect to the advisory vote on the frequency of future advisory votes to approve the compensation of our named executive officers, will be treated as being present at the Annual Meeting for purposes of establishing a quorum. Accordingly, if you are a stockholder of record as of the Record Date and have returned a valid proxy or attend the Annual Meeting in person, your shares will be counted for the purpose of determining whether there is a quorum, even if you wish to abstain from voting on some or all matters at the Annual Meeting. Broker non-votes will also be counted as present for purposes of determining the presence of a quorum.

Required Vote

        With respect to Proposal One, you may vote "FOR" all nominees, "WITHHOLD" your vote as to all nominees, or "FOR" all nominees except those specific nominees from whom you "WITHHOLD" your vote. A properly executed proxy marked "WITHHOLD" with respect to the election of one or more directors will not be voted with respect to the director or directors indicated. Members of the Board are elected by a plurality of votes cast, in person or by proxy, at the Annual Meeting. This means that the eight nominees who receive the greatest number of "FOR" votes cast will be elected. Cumulative voting is not permitted. Neither broker non-votes nor votes marked "WITHHOLD" will have an effect with respect to the election of any nominee.

        You may vote "FOR," "AGAINST" or "ABSTAIN" on Proposals Two and Three. To be approved, each of Proposals Two and Three must receive the affirmative vote of a majority of the votes cast, in person or by proxy, at the Annual Meeting on the proposal. Abstentions and broker non-votes, if any, will not be counted as votes cast on Proposals Two and Three and will have no effect on the result of either vote.

Board Recommendation

        The Board recommends that you vote as follows:

•

"FOR" each of the eight director nominees set forth in Proposal One;



•

"FOR" Proposal Two, relating to the ratification of KPMG as our independent registered public accounting firm for the fiscal year ending December 31, 2018;2019; and



•

"FOR" Proposal Three, relating to the advisory vote to approve the compensation of our named executive officers.

        Any properly authorized proxy as to which no instructions are given will be voted in accordance with the foregoing recommendations.

Table of Contents

Revocation of Proxies

        You may revoke your proxy at any time prior to it being exercised by (i) delivering a written notice of revocation to our Secretary, (ii) submitting a duly executed proxy bearing a later date with us or (iii) attending the Annual Meeting and voting in person. Attendance at the Annual Meeting will not, by itself, revoke a duly executed proxy.

Voting Results

        The voting results will be tallied by the inspector of election appointed for the Annual Meeting and filed with the SEC in a Current Report on Form 8-K within four business days following the Annual Meeting.

Table of Contents

PROPOSAL ONE: ELECTION OF DIRECTORS

        Currently, there are eight directors on our Board. The eight persons named below, each of whom currently serves on our Board, have been recommended by our Nominating/Corporate Governance Committee and nominated by our Board to serve on the Board until our 20192020 Annual Meeting of Stockholders and until their respective successors are elected and qualify. The Board has no reason to believe that any of the persons named below as a nominee for our Board will be unable, or will decline, to serve as a member of the Board if elected. Each of the nominees has consented to being named in this proxy statement. In addition, the Board has determined that all of our directors, other than Paul E. Szurek, our President and Chief Executive Officer, are independent under applicable SEC and NYSE rules. In making their independence determination, the Board considered the relationship of James A. Attwood, Jr. and Robert G. Stuckey with The Carlyle Group L.P. ("Carlyle"), a significant holder of our Operating Partnership.Partnership's units. A plurality of votes cast is necessary for the election of a director. There is no cumulative voting in the election of directors.

        The Nominating/Corporate Governance Committee has not set forth minimum qualifications for Board nominees. However, pursuant to its charter, in identifying candidates to recommend for election to the Board, the Nominating/Corporate Governance Committee considers the following criteria:

•

personal and professional integrity, ethics and values;



•

experience in corporate governance, including as an officer, board member or senior executive or as a former officer, board member or senior executive of a publicly held company, and a general understanding of marketing, finance and other elements relevant to the success of a publicly traded company in today's business environment;



•

experience in our industry and taking into account the interests of our various stockholders;



•

experience as a board member of another publicly held company;



•

academic expertise in an area of our operations;



•

diversity of experience, profession, expertise, skill and background (including with respect to race and gender), both on an individual level and in relation to the Board as a whole;



•

practical and mature business judgment, including the ability to make independent analytical inquiries; and



•

the nature of and time involved in a director's service on other boards of directors and/or committees.

        The Nominating/Corporate Governance Committee continually reviews Board composition and potential additions while striving to maintain and grow a diverse and broad skill set that complements the business. For example, over the past twolast few years, the Board has added Kelly C. Chambliss and Jean A. Bua as members of the Board, and the Board believes these new directors have provided valuable experience and insight, along with additional diversity to the Board. Although the Board does not have a formal policy specifying how diversity of background and personal experience should be applied in identifying or evaluating director candidates, to help ensure that the Board remains aware of and responsive to the needs and interests of our customers, stockholders, employees and other stakeholders, the Board believes it is important to identify otherwise qualified director candidates that would increase the gender, racial, ethnic and/or cultural diversity of the Board. Accordingly, the Nominating/Corporate Governance Committee makes an effort when nominatingevaluating the current directors and considering the nomination of new directors to ensure that the composition of the Board reflects a broad diversity of experience, profession, expertise, skill, and background, including gender, racial, ethnic, and/or cultural diversity. The Nominating/Corporate Governance Committee does not assign a specific weight to the various factors it considers in evaluating potential new candidates to the Board, and no particular criteria is necessarily applicable to all prospective nominees. In the evaluation of

Table of Contents

to all prospective nominees. In the evaluation of potential new candidates, the Nominating/Corporate Governance Committee considers each candidate's qualifications in light of the then-current mix of Board attributes, including diversity. Continuing directors are evaluated by the Nominating/Corporate Governance Committee in the same way, including the continuing director's past contributions to the Board in such evaluation.

        In identifying potential candidates for the Board, other than the director candidate nominated by Carlyle, the Nominating/Corporate Governance Committee generally relies on a variety of resources to identify potential candidates, which, among other things and depending on the circumstances, may include its and the Board's network of contacts, corporate search resources, and, if the Nominating/Corporate Governance Committee deems appropriate, a professional search firm. By utilizing a broad variety of resources as deemed appropriate by the Nominating/Corporate Governance Committee in light of the then-current mix of Board attributes and any previously identified potential candidates, the Nominating/Corporate Governance Committee believes it will be able to identify, evaluate and consider a diverse range of qualified candidates, including candidates that increase the gender, racial, ethnic and/or cultural diversity of the Board.

        Under the partnership agreement governing our Operating Partnership (the "Operating Partnership Agreement"), Carlyle, which directly or indirectly holdsowns approximately 28.6% ownership of23.9% our Operating Partnership, is currently entitled to nominate one director for election to our Board. (SeeSee "Information about our Board and its Committees" for more information about the circumstances under which Carlyle is entitled to appoint nomineesa nominee to our Board.) Carlyle has exercised this right by nominating Robert G. Stuckey for election at the Annual Meeting.

        James A. Attwood, Jr. was originally nominated to our Board pursuant to Carlyle's previous right to nominate two directors for election to our Board. In May 2015, Carlyle's right to nominate two directors was decreased to one director due to the decrease in Carlyle's ownership percentage of our Operating Partnership. Upon consideration of the contributions of Mr. Attwood to our Board, our Nominating/Corporate Governance Committee and the Board approved the continued nomination of Mr. Attwood for election as a director. Mr. Attwood was elected by approximately 98.6%98.3% of the votes cast in the election of directors at the 20172018 Annual Meeting of Stockholders. Taking into account Mr. Attwood's continued relationship with Carlyle, the Nominating/Corporate Governance Committee and the Board continue to believe that Mr. Attwood is a valuable member of the Board due to, among other things, his experience with our Company and in our industry, and the Board, based on the recommendation of the Nominating/Corporate Governance Committee, has nominated Mr. Attwood to be elected as a director at the Annual Meeting.

Nominees for Election as Directors

        The table below sets forth the names and biographical information of each of the directors nominated for election at the Annual Meeting.

Table of Contents

Directors

        Robert G. Stuckey is our Chairman of the Board and has served as a director since September 2010. Mr. Stuckey is a Managing Director and Fund Head, US Real Estate, at Carlyle. Prior to joining Carlyle in 1998, Mr. Stuckey was Chief Investment Officer at CarrAmerica Realty Corporation, ("CarrAmerica"), a real estate investment trust ("REIT"). Prior to that, he was Senior Vice President of Prologis, Inc. ("Prologis"), a REIT which is an owner, operator and developer of industrial real estate, and Chief Financial Officer for Trammell Crow Company (Northeast Region), a developer of, and investor in, commercial real estate. Mr. Stuckey was twice an Academic All-American in football at the University of Nebraska and received an M.B.A. from Harvard University. In determining Mr. Stuckey's qualifications to serve on our Board, the Board considered, among other things, Mr. Stuckey's significant experience concerning the acquisition, disposition, financing, operations and market opportunities of data center properties and private and publicly traded REITs, which provides us with valuable insight into commercial real estate, REIT and data center industry trends that affect our business.

        Paul E. Szurek has served as our President and Chief Executive Officer since September 2016, and as a director since September 2010. From 2003 to August 2016, Mr. Szurek was Chief Financial Officer of Biltmore Farms, LLC ("Biltmore Farms"), a residential and commercial real estate development and operating company. Prior to joining Biltmore Farms, LLC, Mr. Szurek served as Chief Financial Officer of Security Capital Group Incorporated, a publicly traded real estate investment, development and operating company with extensive REIT engagement. Mr. Szurek is currently a director of Four Corners Property Trust, a REIT focused on restaurant real estate. He has also previously served as a director of the Charlotte, North Carolina branch of the Federal Reserve Bank of Richmond and as a director to two publicly traded real estate companies, Regency Centers and Security Capital U.S. Realty. Mr. Szurek received a J.D. with honors from Harvard Law School and a B.A. in Government, magna cum laude, from the University of Texas at Austin. In determining Mr. Szurek's qualifications to serve on our Board, the Board considered, among other things, Mr. Szurek's significant experience concerning the acquisition, disposition, financing, operations and market opportunities of private and publicly traded REITs, which provide us with valuable insight into REIT-industry trends that affect our business. In addition, Mr. Szurek provides valuable insight to the Board based on the day-to-day operations and issues he encounters as our President and Chief Executive Officer.

James A. Attwood, Jr.    has served as a director since September 2010. Mr. Attwood is a Managing Director and the former Head of the Global Telecommunications, Media and Technology Group at Carlyle. Prior to joining Carlyle in 2000, Mr. Attwood served as Executive Vice President for Strategy, Development and Planning at Verizon Communications, Inc. ("Verizon"), a telecommunications provider, and GTE Corporation (prior to its merger with Bell Atlantic to form Verizon). Prior to his four years at Verizon and GTE Corporation, Mr. Attwood served as an investment banker at Goldman, Sachs & Co. for 11 years. Mr. Attwood graduated summa cum laude from Yale University with a B.A. in applied mathematics and an M.A. in statistics. He also received J.D. and M.B.A. degrees from Harvard University. Mr. Attwood currently serves as a member of the boards of directors of Syniverse Holdings, Inc., a privately held provider of business and technology services for the mobile telecommunications industry, and Nielsen Holdings plc, a global information and measurement company, and Getty Images Inc., a global creator and distributor of still imagery, video and multimedia products, which is privately held.company. Mr. Attwood has gained significant knowledge of the telecommunications industry through his work with Verizon and Carlyle. In determining Mr. Attwood's qualifications to serve on our Board, the Board considered, among other things, Mr. Attwood's private equity experience, together with the experience gained by having served as an officer and on the boards of directors of various telecommunications companies, which provide us with a valuable perspective in monitoring and evaluating our business.

Table of Contents

        Jean A. Bua has served as a director since May 2017. Ms. Bua has served as Executive Vice President, Chief Financial Officer, Chief Accounting Officer and Treasurer of NETSCOUT,NetScout Systems, Inc. ("NetScout"), a provider

Table of Contents

of real-time operational intelligence and performance analytics for service assurance and cyber security solutions, since September 2015 and previously served as NETSCOUT'sNetScout's Senior Vice President, Chief Financial Officer, Chief Accounting Officer and Treasurer from November 2011 until September 2015. She originally joined NETSCOUTNetScout in September 2010 as Vice President, Finance. Prior to NETSCOUT,NetScout, Ms. Bua served as Executive Vice President, Finance & Treasurer of American Tower Corporation, an operator of wireless and broadcast communications infrastructure, and spent nine years at Iron Mountain, Inc., an information management services company, concluding as Senior Vice President, Chief Accounting Officer and Worldwide Controller. She also previously held senior positions at Duracraft Corp. and Keithley Instruments and was a management consultant at Ernst & Young LLP and an auditor at KPMG LLP. Ms. Bua is currently a member of the board of directors of AstroNova, Inc., a provider of data visualization technologies. Ms. Bua earned a Bachelor of Science in Business Administration, summa cum laude, from Bryant College and a Masters of Business Administrationan M.B.A. from the University of Rhode Island. In determining Ms. Bua's qualifications to serve on our Board, the Board considered, among other things, Ms. Bua's significant industry experience in the areas of accounting policy, internal controls and risk management.

        Kelly C. Chambliss has served as a director since September 2016. Ms. Chambliss has been the General Manager and Managing Partner of the Distribution and Industrial Sectors within IBM Global Business Services (GBS) North America since May 2015, with the Industrial Sector added in June 2017.April 2018. Prior to this role, frombeginning in October 2013, to April 2015, Ms. Chambliss served as the Global Chief Technology Officer (CTO) for IBM GBS, with a focus on the development, marketing, sales, and delivery of cloud-based solutions. Ms. Chambliss served in various other roles at IBM from late 2002 to October 2013. Ms. Chambliss joined IBM in late 2002 through the acquisition of PricewaterhouseCoopers (PwC) Consulting, where she was a Partner. Ms. Chambliss graduated from Virginia Tech with a degree in Management Science and a specialization in Decision Support Systems. In determining Ms. Chambliss' qualifications to serve on our Board, the Board considered, among other things, Ms. Chambliss' significant experience in the technology and technology consulting industries, which provides us with valuable insight into the technology and market trends that affect our industry.

        Michael R. Koehler has served as a director since September 2010. Mr. Koehler currently serves as Senior Vice President and Chief Information and Chief Digital Officer of Exelon Corporation, an electric and gas utility company, which he joined in April 2016. From September 2015 to April 2016, he served as Managing Partner of Volution Partners L.L.C., a management consulting firm specializing in information technology. Mr. Koehler served as Senior Vice President, Global Services, of EMC Corporation, a provider of ITinformation technology consulting services and hardware, from August 2011 to September 2015. During 2008 and 2009, Mr. Koehler served as Senior Vice President, Americas Region, of Electronic Data Systems Corporation ("EDS"), a division of the Hewlett-Packard Company ("HP"). EDS, a global provider of information technology and business processing outsourcing services, was acquired by HP in 2008. Prior to HP's acquisition of EDS, Mr. Koehler served as Executive Vice President, Global ITO Services and as Senior Vice President, Infrastructure Technology and Business Process Outsourcing at EDS. During 2007, Mr. Koehler served as Regional Senior Vice President, Europe, Middle East and Africa Operations at EDS, and, from 2006 to 2008, as Enterprise Client Executive, Navy Marine Corps Intranet Account at EDS. From 2004 to 2006, Mr. Koehler served as Chief Operating Officer of The Feld Group, a management information technology consulting firm that was acquired by EDS in 2004. From 1994 to 2001, he held management positions of increasingly greater responsibility at The Feld Group. Mr. Koehler received his B.S. in Industrial Engineering from Texas Tech University. In determining Mr. Koehler's qualifications to serve on our Board, the Board considered, among other things, Mr. Koehler's significant experience in the technology consulting and

Table of Contents

outsourcing industries and extensive operational and strategic planning experience in complex global companies, which provide us with valuable insight into the technology trends that affect our business.

Table of Contents

        J. David Thompson has served as a director since September 2010. Mr. Thompson has been Chief Information and Technology Officer of American Express Global Business Travel, a multinational travel and meetings program management company, since October 2017. From April 2012 to October 2017, he was Executive Vice President and Chief Information Officer of The Western Union Company, a global payment services company. From 2006 to April 2012, Mr. Thompson was Group President of the Symantec Services Group and Chief Information Officer of Symantec Corporation, a global provider of security, storage, and systems management solutions. From 2004 to 2006, Mr. Thompson served as Senior Vice President and Chief Information Officer for Oracle Corporation. Mr. Thompson was Senior Vice President and Chief Information Officer at PeopleSoft, Inc. from 1998 to 2005, prior to its acquisition by Oracle Corporation. Mr. Thompson began his career as an officer in the U.S. Air Force as an Intelligence Systems Officer. Mr. Thompson studied computer science at American University. In determining Mr. Thompson's qualifications to serve on our Board, the Board considered, among other things, Mr. Thompson's significant experience in the technology industry and extensive operational experience in information technology systems optimization, which provide us with valuable insights into the information technology trends that affect our business.

        David A. Wilson has served as a director since September 2010. Mr. Wilson served as President and Chief Executive Officer of the Graduate Management Admission Council (the "Council") from 1995 until his retirement in December 2013. The Council is the owner of the Graduate Management Admission Test, the GMAT. Prior to 1995, he was a Managing Partner and National Director for Professional Development at Ernst & Young LLP, a public accounting firm. From 1968 to 1978, Mr. Wilson held faculty positions at the University of Texas at Austin, where he was awarded tenure, and at Harvard Business School. Mr. Wilson completed his undergraduate studies at Queen's University in Canada, received his M.B.A. at the University of California, Berkeley, and received his doctorate at the University of Illinois. He is a Chartered Accountant in Canada and a Certified Public Accountant in the United States. Mr. Wilson served on the board of directors of Laureate Education, Inc. ("Laureate"), a company which provides access to high-quality, innovative institutions of higher education, from 2002 to 2007, and of Terra Industries, Inc. ("Terra"), a producer and marketer of nitrogen products for the agricultural and industrial markets, from 2009 to 2010. At Laureate, he chaired the Audit Committee and served as a member of the Nominating and Governance Committee and the Conflicts Committee. He served on the Audit Committee of Terra. He has served on the Worldwide Board of Junior Achievement, the Conseil d'Administration de la Confrérie de la Chaîne des Rôtisseurs (Paris) and The Wolf Trap Foundation. Mr. Wilson served on the board of directors of Barnes and Noble, Inc., a book retailer and content, commerce and technology company, and as chair of its Audit Committee from September 2010 to August 2015, and currently serves on the board of directors of Barnes & Noble Education, Inc., a provider of educational services, and as chair of its Audit Committee. He serves on the Board of Trustees of Johnson & Wales University and as Chairman of its Audit Committee and a member of its Finance Committee. Mr. Wilson is a National Association of Corporate Directors (NACD) GovernanceLeadership Fellow and received the CERT Certificate in Cybersecurity Oversight awarded by the Software Engineering Institute of Carnegie Mellon University. He was also a 2015 honoree of the NACD Directorship 100 award. In determining Mr. Wilson's qualifications to serve on our Board, the Board considered, among other things, Mr. Wilson's significant industry experience in the areas of accounting policy, internal controls and risk management.

Table of Contents

Table of Contents

PROPOSAL TWO: RATIFICATION OF THE APPOINTMENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

        The Audit Committee of our Board, which is composed entirely of independent directors, has appointed KPMG as our company's independent registered public accounting firm for the fiscal year ending December 31, 2018.2019. KPMG has been our independent registered public accounting firm since our initial public offering ("IPO") in 2010. Although stockholder approval is not required, we desire to obtain from our stockholders an indication of their approval or disapproval of the Audit Committee's action in appointing KPMG as the independent registered public accounting firm of our company for 2018.2019. If our stockholders do not ratify and approve this appointment, the appointment will be reconsidered by the Audit Committee and our Board.

        A representative of KPMG will be present at our Annual Meeting, where the representative will be afforded an opportunity to make a statement and to respond to appropriate questions.

        The affirmative vote of a majority of all votes cast on the proposal is necessary to ratify the appointment of KPMG as our independent registered public accounting firm for the fiscal year ending December 31, 2018.2019.

Annual Evaluation and Selection of KPMG

        The Audit Committee annually evaluates the performance of its independent registered public accounting firm, including the senior members of the audit engagement team, and determines whether to reengage the current independent auditors or consider other audit firms. Factors considered by the Audit Committee in deciding whether to retain include:

•

KPMG's capabilities considering the complexity of our business, and the resulting demands placed on KPMG in terms of technical expertise and knowledge of our industry and business;



•

the quality and candor of KPMG's communications with the Audit Committee and management;



•

KPMG's independence;



•

the quality and efficiency of the services provided by KPMG, including input from management on KPMG's performance and how effectively KPMG demonstrated its independent judgment, objectivity and professional skepticism;



•

external data on audit quality and performance, including recent Public Company Accounting Oversight Board (PCAOB)("PCAOB") reports on KPMG and its peer firms; and



•

the appropriateness of KPMG's fees, tenure as our independent auditor, including the benefits of a longer tenure, and the controls and processes in place that help ensure KPMG's continued independence.

        Based on this evaluation, the Audit Committee and the Board believe that retaining KPMG to serve as our independent registered public accounting firm for the fiscal year ending December 31, 2018,2019, is in the best interests of our Companycompany and its stockholders.

        The Audit Committee also oversees the process for, and ultimately approves, the selection of our independent registered public accounting firm's lead engagement partner at the five-year mandatory rotation period. Fiscal year 2016 wasPrior to the fifth year KPMG's lead engagement partner served on the Company's audit. Atmandatory rotation period, at the Audit Committee's instruction, KPMG selectedwill select candidates and providedprovide qualifications to be considered for the lead engagement partner role, who werewill then be interviewed by members of our Audit Committee and senior management. After considering the candidates and their qualifications recommended by KPMG, senior management and the Audit Committee discusseddiscuss the candidates and their relative qualifications. The Audit Committee then discussed the candidates with the current lead engagement partner, interviewed the leading candidate, and ultimately approved thiswill

Table of Contents

then discuss the candidates with the current lead engagement partner, interview the leading candidate, and ultimately approve the individual. The most recent new KPMG lead engagement partner commenced service on our company's audit in 2017.

Principal Accountant Fees and Services

        The following table summarizes the fees billed by KPMG for professional services rendered to us for fiscal years 20162018 and 2017.

Audit Fees.    Audit fees consisted of aggregate fees billed for professional services rendered for the audit of our consolidated annual financial statements, review of interim consolidated financial statements, consultations on accounting matters directly related to the audit, or services that are normally provided by the independent registered public accounting firm in connection with statutory and regulatory filings or engagements.

Audit-Related Fees.    Audit-related fees consisted of aggregate fees billed for accounting consultations and other services that were reasonably related to the performance of audits or reviews of our financial statements and were not reported above under "Audit Fees." The amountsamount for 2016 related2018 relates to comfort letter services provided in connection with the sale of shares by the Carlyle Funds in a registered public offerings.offering.

Pre-Approval Policies and Procedures

        The Audit Committee pre-approves all audit and non-audit services provided by our independent registered public accounting firm. For each proposed service, the independent registered public accounting firm is required to provide detailed supporting documentation at the time of approval to permit the Audit Committee to make a determination as to whether the provision of such services would impair the independent registered public accounting firm's independence, and whether the fees for the services are appropriate. The Audit Committee pre-approved all services performed by, and audits fees paid to, our independent registered public accounting firm during fiscal years 20162018 and 2017.

Table of Contents

AUDIT COMMITTEE REPORT

        The Audit Committee (the "Audit Committee") of the Board of Directors (the "Board") of CoreSite Realty Corporation (the "Company") assists the Board with its oversight responsibilities regarding the Company's financial reporting process. The Company's management is responsible for the preparation, presentation and integrity of the Company's financial statements and the reporting process, including the Company's accounting policies, internal audit function, internal control over financial reporting and disclosure controls and procedures. KPMG LLP, the Company's independent registered public accounting firm, is responsible for performing an audit of the Company's financial statements.

        With regard to the fiscal year ended December 31, 2017,2018, the Audit Committee (i) reviewed and discussed with management our audited consolidated financial statements as of December 31, 2017,2018, and for the year then ended; (ii) discussed with KPMG LLP, the independent auditors, the matters required by PCAOB AS Section 1301,Communications with Audit Committees; (iii) received the written disclosures and the letter from KPMG LLP required by applicable requirements of the PCAOB regarding KPMG LLP's communications with the Audit Committee regarding independence; and (iv) discussed with KPMG LLP their independence.

        Based on the review and discussions described above, the Audit Committee recommended to ourthe Board that ourthe Company's audited financial statements be included in ourits Annual Report on Form 10-K for the fiscal year ended December 31, 2017,2018, for filing with the Securities and Exchange Commission.

The Audit Committee:

David A. Wilson (Chair)
Jean A. Bua
Michael R. Koehler
David A. Wilson

Table of Contents

PROPOSAL THREE: ADVISORY VOTE TO APPROVE THE COMPENSATION OF OUR
NAMED EXECUTIVE OFFICERS

        Pursuant to Section 14A of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), our stockholders are entitled to cast an advisory vote at the Annual Meeting to approve the compensation of our named executive officers, as disclosed pursuant to the SEC's compensation disclosure rules, including the Compensation Discussion and Analysis section of this proxy statement, or CD&A, the compensation tables and accompanying narrative disclosures. While this stockholder vote on executive compensation is an advisory vote that is not binding on our company or the Board, we value the opinions of our stockholders and will consider the outcome of the vote when making future compensation decisions. The advisory vote to approve the compensation of our named executive officers requires the affirmative vote of a majority of all the votes cast on the proposal. Our current policy is to provide stockholders with an opportunity to approve the compensation of our named executive officers each year at the annual meeting of stockholders until the next required stockholder vote on the frequency of such votes. We expect that the next such frequency vote will occur at the 2023 Annual Meeting of Stockholders.

        As described more fully in the CD&A, ourOur executive compensation program is designed to attract, motivate and retain individuals with the skills required to formulate and drive our strategic direction and achieve the annual and long-term performance necessary to create stockholder value. The program also seeks to align executive compensation with stockholder value on an annual and long-term basis through a combination of base pay, annual incentives and long-term incentives. Our practice of placing a significant portion of each executive's compensation at risk demonstrates this pay-for-performance philosophy.

        We actively review and assess our executive compensation program in light of the industry in which we operate, the marketplace for executive talent in which we compete, and evolving compensation governance and best practices. We are focused on compensating our executive officers fairly and in a manner that promotes our compensation philosophy and is consistent with our annual and longer-term performance. Specifically, our compensation program for executive officers focuses on the following principal objectives:

•

align executive compensation with stockholder interests;



•

attract and retain talented personnel by offering competitive compensation packages;



•

motivate employees to achieve strategic and tactical corporate objectives and the profitable growth of our company; and



•

reward employees forreinforce a strong performance-oriented environment in the payment of executive compensation based on achievement of annual and longer-term milestones and individual functional and corporate performance.contributions within a team culture.

        Our Board believes that our executive compensation program satisfies these objectives, properly aligns the interests of our executive officers with those of our stockholders, and is worthy of stockholder support. In determining whether to approve this proposal, we believe that stockholders should consider the following:

•

Independent Compensation Committee.  Executive compensation is reviewed and established by our Compensation Committee consisting solely of independent directors. The Compensation Committee meets in executive session in determining annual compensation. The Compensation Committee receives data, analysis and input from an independent compensation consultant.



•

Performance-Based Incentive Compensation.  Elements of performance-based, incentive compensation are largely aligned with financial and operational objectives established in the Board-approved annual operating plan.

Table of Contents

•

Limited Perquisites.  Our executive officers receive minimal perquisites, consisting primarily of group term life insurance premiums.



•

Equity Plans.  Grants under our equity plans generally include time-based and performance-based vesting periods, and our plan prohibits repricing or exchange of outstanding option awards without consent of stockholders, and requires that options be granted with exercise prices at fair market value.

        Accordingly, we ask our stockholders to vote "FOR" the following resolution at the Annual Meeting:

        "RESOLVED, that the stockholders approve, on an advisory basis, the compensation of our named executive officers, as disclosed pursuant to Item 402 of Regulation S-K, including the Compensation Discussion and Analysis, the compensation tables and narrative discussion in this proxy statement."

Table of Contents

INFORMATION ABOUT OUR BOARD OF DIRECTORS AND CORPORATE GOVERNANCE

        Our Board currently consists of eight directors. Our charter and bylaws provide that the number of directors constituting our Board may be increased or decreased by a majority vote of our entire Board, provided the number of directors may not be decreased to fewer than one, the minimum number required under the Maryland General Corporation Law nor, unless our bylaws are amended, more than 15 directors.

        Our bylaws require that nominees for director, whether for election by the stockholders or by the Board, shall include such individuals as are entitled to be nominated pursuant to the Operating Partnership Agreement. The Operating Partnership Agreement provides that, for so long as the number of Operating Partnership units and shares of common stock held collectively by the real estate funds affiliated with Carlyle (the "Carlyle Funds") is less than 50% but at least 10% of the total number of shares of our outstanding common stock (assuming all Operating Partnership units are exchanged for common stock), certain of the Carlyle Funds shall have the right to nominate the number of directors that is one less than the lowest whole number that would exceed 20% of the directors, but not less than one director. With the Board currently consisting of eight members,directors, the Carlyle Funds presently are entitled to nominate one director. Such rights to nominate directors are subject to increase or decrease as follows (in each case assuming all Operating Partnership units are exchanged for common stock):

•

If the Carlyle Funds collectively own equal to or greater than 50% of theour outstanding common stock, then the Carlyle Funds would have the right to nominate the number of directors that is one less than the lowest whole number that would exceed one-third of the directors, but not less than one director. Assuming the Board still had eight directors, then the Carlyle Funds would be entitled to nominate two directors under this scenario.



•

If the Carlyle Funds collectively own less than 10% of the outstanding common stock, then the Carlyle Funds would no longer be entitled to nominate any directors.

Board and Committee Meetings

        During 2017,2018, the Board held sixfive meetings. Each member of the Board attended or participated in 75% or more of the aggregate of (i) the total number of meetings of the Board (held during the period for which such person has been a director) and (ii) the total number of meetings held by all committees of the Board on which such person served (during the periods that such person served).

Board Leadership Structure

        The Board does not have a policy regarding separation of the roles of Chief Executive Officer and Chairman of the Board. The Board believes it is in our best interests to make that determination based on current circumstances. The Board has determined that an independent director serving as Chairman is in our best interests at this time. The current Chairman of the Board is Robert G. Stuckey. This structure ensures a greater role of independent directors in the active oversight of our business, including risk management oversight, and in setting agendas and establishing Board priorities and procedures. This structure also allows our Chief Executive Officer to focus to a greater extent on the management of our day-to-day operations. In addition to an independent chairman, our bylaws also require us to have a lead independent director, who may preside over meetings of independent directors in the event of a potential conflict of interest that precludes our chairman from participating. Our current lead independent director is David A. Wilson.

Table of Contents

Executive Sessions

        Our non-employee directors met in a special executive session without management at each regularly scheduled Board meeting in 2017.2018, except for the Board meeting held in February 2018. Robert G. Stuckey, as Chairman of the Board, chaired the non-employee director executive sessions held during 20172018, and generally chairs any executive sessions held by non-employee directors. The Board expects to continue to conduct an executive session limited to non-employee directors at least annually and our non-employee directors may schedule additional executive sessions in their discretion.

Committees of the Board of Directors

        Our Board has a standing Audit Committee, Compensation Committee and Nominating/Corporate Governance Committee. Each of these committees must be composed exclusively of independent directors. The Audit Committee and the Nominating/Corporate Governance Committee must each have at least three directors and the Compensation Committee must have at least two directors. Our Board may from time to time establish other committees to facilitate the management of our company. The Operating Partnership Agreement currently requires that, so long as the Carlyle Funds collectively own at least 10% of theour outstanding common stock (assuming all Operating Partnership units are exchanged for common stock), the Carlyle Funds shall have the right to have at least one of their nominees on each committee, unless prohibited by law or the rules of the NYSE, other than any committee whose purpose is to evaluate or negotiate any transaction with the Carlyle Funds. The Carlyle Funds have exercised this right by requesting that Mr. Stuckey be appointed to the Nominating/Corporate Governance Committee, but have not requested that Mr. Stuckey be appointed to either the Audit Committee or the Compensation Committee.

        The following table is a summary of our committee structure and members on each of our committees:

        The Audit Committee, Compensation Committee and Nominating/Corporate Governance Committee each operate under written charters adopted by the Board. These charters are available on our website at www.coresite.com. The information contained on our website is not part of, or incorporated by reference in, this proxy statement.

Audit Committee

        The Audit Committee assists the Board with its oversight responsibilities regarding the integrity of our financial statements, the qualifications and independence of our independent auditor and the performance of our internal audit function and independent auditors. The Audit Committee selects,

Table of Contents

appoints, assists and meets with the independent auditor, oversees each annual audit and quarterly

Table of Contents

review, establishes and maintains our internal audit controls and prepares an annual report for inclusion in our annual proxy statement pursuant to federal securities laws. The Audit Committee is specifically responsible for oversight and evaluation of our independent auditor. The Audit Committee also works with management regarding risk assessment and risk management (including cybersecurity and other risks relevant to our computerized information system controls and security), and discusses with management our leverage and any issues that arise with respect to our leverage. The Board has determined that each member of the Audit Committee is "financially literate" as defined under the NYSE rules, that each of Ms. Bua and Mr. Wilson qualifies as an "Audit Committee Financial Expert""audit committee financial expert" as defined under SEC rules, and that each member of the Audit Committee is independent under applicable NYSE and SEC rules for purposes of membership on the Audit Committee. The Audit Committee met eightseven times in 2017.2018.

Nominating/Corporate Governance Committee

        The Nominating/Corporate Governance Committee assists the Board in identifying qualified individuals to become directors, makes recommendations to the Board concerning the size, structure and composition of the Board and its committees, monitors the process to assess the Board's effectiveness and is primarily responsible for oversight of corporate governance, including implementing our Corporate Governance Guidelines. The Nominating/Corporate Governance Committee also oversees the development of a succession plan in the event of our Chief Executive Officer's retirement or an unexpected event, which it reviews on an annual basis.

        In evaluating potential nominees to the Board, the Nominating/Corporate Governance Committee considers, among other things, independence, character, ability to exercise sound judgment, demonstrated leadership, skills, including financial literacy, and experience in the context of the needs of the Board, as well as criteria for Board nominees set forth under "Proposal One: Election of Directors" above. The Nominating/Corporate Governance Committee considers candidates proposed by stockholders and evaluates them using the same criteria as for other candidates. The Nominating/Corporate Governance Committee considers all recommended director candidates submitted to it in accordance with the established procedures, though it will only recommend to the Board as potential nominees those candidates it believes are most qualified. However, the Nominating/Corporate Governance Committee will not consider any director candidate if the candidate's candidacy or, if elected, Board membership, would violate controlling state or federal law. Any stockholder wishing to recommend a candidate for consideration by the Nominating/Corporate Governance Committee may do so by submitting a written recommendation to the Nominating/Corporate Governance Committee in accordance with the procedures set forth under "Miscellaneous—Stockholder Proposals and Nominations."

        The Board has determined that each member of the Nominating/Corporate Governance Committee is independent under applicable NYSE rules. The Nominating/Corporate Governance Committee met fourfive times in 2017.2018.

        At least annually, the Nominating/Corporate Governance Committee evaluates the performance of each current director and considers the results of such evaluation when determining whether to recommend the nomination of such director for an additional term. At an appropriate time prior to each annual meeting at which directors are to be elected or re-elected, the Nominating/Corporate Governance Committee recommends to the Board for nomination by the Board such candidates as the Nominating/Corporate Governance Committee, in the exercise of its judgment, has found to be well qualified and willing and available to serve.

Table of Contents

        At an appropriate time after a vacancy arises on the Board or a director advises the Board of his or her intention to resign, the Nominating/Corporate Governance Committee will recommend to the Board for election by the Board to fill such vacancy, such prospective member of the Board as the Nominating/Corporate Governance Committee, in the exercise of its judgment, has found to be well qualified and willing and available to serve. In determining whether a prospective member is qualified to serve, the Nominating/Corporate Governance Committee will consider the factors listed above.

Table of Contents

        The foregoing notwithstanding, if we are legally required by contract or otherwise to permit a third party to designate one or more of the director nominees to be elected (for example, pursuant to rights contained in the Operating Partnership Agreement), then such legal requirements will be taken into consideration by the Nominating/Corporate Governance Committee.

Compensation Committee

        The Compensation Committee reviews and approves the compensation and benefits of our executive officers, administers and makes recommendations to the Board regarding our compensation and stock incentive plans, produces an annual report on executive compensation for inclusion in our annual proxy statement and publishes an annual committee report for our stockholders. The Board has determined that each member of the Compensation Committee is independent under applicable NYSE and SEC rules for purposes of membership on the Compensation Committee. The Compensation Committee met four times in 2017.2018. For a description of the Compensation Committee's processes and procedures, including the roles of our executive officers and independent compensation consultants in the Compensation Committee's decision-making process, see the section titled "Compensation Discussion and Analysis."

Board Oversight of Risk Management

        The Board believes that overseeing how the executive team manages the various risks confronting the company is one of its most important areas of oversight. In carrying out this critical responsibility, the Board has designated the Audit Committee with primary responsibility for overseeing enterprise risk management (including cybersecurity and other risks relevant to our computerized information system controls and security). While the Audit Committee has primary responsibility for overseeing enterprise risk management, each of the other Board committees also considers risk within its area of responsibility. For example, the Nominating/Corporate Governance Committee reviews risks related to legal and regulatory compliance as they relate to corporate governance structure and processes, and the Compensation Committee reviews risks related to compensation matters. The Board is apprised by the committee chairs of significant risks and management's response to those risks via periodic reports. While the Board and its committees oversee risk management strategy, management is responsible for implementing and supervising day-to-day risk management processes and reporting to the Board and its committees on such matters.

        With respect to risk related to compensation matters, the Compensation Committee considers, in establishing and reviewing our executive compensation program, whether the program encourages unnecessary or excessive risk taking and has concluded that it does not. Executives' base salaries are fixed in amount and thus do not encourage risk-taking. Bonuses are capped and are tied to overall corporate performance. Any other compensation provided to our executive officers is primarily in the form of long-term equity awards that are important to help further align executives' interests with those of our stockholders. The Compensation Committee believes that these awards do not encourage unnecessary or excessive risk-taking because the ultimate value of the awards is tied to our stock price and because awards are staggered and subject to long-term vesting schedules to help ensure that executives have significant value tied to long-term stock price performance.

Table of Contents

        The Compensation Committee also has reviewed our compensation programs for employees generally and has concluded that these programs do not create risks that are reasonably likely to have a material adverse effect on the company. The Compensation Committee believes that the design of our annual cash and long-term equity incentives provides an effective and appropriate mix of incentives to help ensure our performance is focused on long-term stockholder value creation and does not encourage the taking of short-term risks at the expense of long-term results. In general, bonus opportunities for our employees are capped, and we have discretion to reduce bonus payments (or pay

Table of Contents

no bonus) based on individual performance and any other factors we may determine to be appropriate in the circumstances.

Evaluation of Board Performance

        The Nominating/Corporate Governance Committee coordinates an annual evaluation process by which the directors evaluate the Board's and the Board committees' performance and procedures. This self-evaluation leads to a full Board discussion of the results. Each committee of the Board also annually evaluates its performance and procedures.

Corporate Governance Guidelines

        The Board has adopted a set of governance guidelines, the CoreSite Realty Corporation Corporate Governance Guidelines, which are designed to promote the continued vitality of the Board and excellence in the execution of its duties. Our Corporate Governance Guidelines establish the practices and procedures of the Board with respect to Board composition and member selection, Board independence, Board meetings and involvement of senior management, management succession planning, Board committees and the evaluation of senior management and the Board. The Board reviews our Corporate Governance Guidelines at least annually and updates them as necessary to reflect improved corporate governance practices and changes in regulatory requirements. A copy of the Corporate Governance Guidelines is available on our website at www.coresite.com.

Stockholder Power to Amend Bylaws

        In March 2017, the Board approved and adopted an amendment and restatement of our bylaws in order to provide our stockholders, to the same extent as the Board, the power to amend the bylaws if such amendment is approved by a majority of all the votes entitled to be cast on the matter. As is the case for Board approved amendments to our bylaws, certain amendments to the bylaws opting out of the Maryland control share acquisition act and relating to board and committee designation rights of Carlyle also require the approval of a majority of the directors who were nominated by Carlyle in accordance with our bylaws. A copy of our bylaws can be obtained from our Secretary, who can be reached at 1001 17th Street, Suite 500, Denver, Colorado 80202.

Stock Ownership Guidelines

        Effective January 1, 2014, the Board adopted stock ownership and retention guidelines for all of our executive officers and non-employee directors who are also not employees of Carlyle ("Non-Employee Directors") to further align their interests with our stockholders. The stock ownership and retention guidelines do not apply to directors who are employed by Carlyle because those directors do not receive compensation for their service as directors. Under these stock ownership and retention

Table of Contents

guidelines, each of our executive officers is expected to own common stock of our company with a market value equal to the following amounts for as long as he or she remains an executive officer:

        Each executive officer and Non-Employee Director must hold 50% of all net settled shares received from the vesting, delivery or exercise of equity awards until such executive officer's or

Table of Contents

Non-Employee Director's stock ownership equals or exceeds the applicable ownership threshold. Executive officers and Non-Employee Directors have until the later to occur of (i) January 1, 2019 or (b) the fifth anniversary of her or his first appointment or election as an executive officer or Non-Employee Director, as applicable, to meet the requirements of these stock ownership guidelines. Once the stock ownership guidelines are met by an executive officer or Non-Employee Director, any subsequent decreases in the market value of our common stock will not be considered for purposes of determining compliance with these stock ownership guidelines with respect to such executive officer's or Non-Employee Director's initial ownership threshold. These stock ownership guidelines were based on analysis of peer and market practices, as prepared by our independent compensation consultant.

Code of Ethics

        Our Code of Business Conduct and Ethics applies to all of our employees, including our principal executive officer, principal financial officer and principal accounting officer, and the Board. A copy of the Code of Business Conduct and Ethics is available on our website at www.coresite.com. We intend to disclose any changes in or waivers from the Code of Business Conduct and Ethics that are required to be disclosed by posting such information on our website.

Anti-Hedging and Pledging Policy - Prohibition on Short Sales, Hedging and Margin Accounts

        Our Insider Trading Policy prohibits our officers, directors and all other employees from (i) engaging in short sales, (ii) buying or selling put or call positions in our securities, (iii) buying financial instruments designed to hedge or offset any decrease in the market value of our securities, and (iv) frequent trading of our securities to take advantage of fluctuations in share price. In addition, all of our officers and directors are prohibited from pledging, purchasing or selling our securities in margin accounts.

20172018 Director Compensation

        Our director compensation program was designed in consultation with the Compensation Committee's independent compensation consultant, including an analysis of peer and market practices.

Table of Contents

Consistent with the previous year, during 2017,2018, our director compensation program consisted of the following:

Table of Contents

        Directors who are employees of our company or our subsidiaries and those directors employed by Carlyle do not receive compensation for their service as directors. During 2017,2018, Messrs. Stuckey, Attwood and Szurek did not receive any compensation for their service on the Board.

        The following table presents information regarding the compensation paid during 20172018 to Non-Employee Directors who served on the Board during the year.

Table of Contents

        The following table presents the number of outstanding and unexercised option awards and the number of outstanding RSUs held by each of the Non-Employee Directors as of December 31, 2017.2018.

Communications with the Board

        Any stockholder or other interested party may contact the Board, including any non-employee director or the non-employee directors as a group, or any individual director or directors, by writing to our Secretary at 1001 17th Street, Suite 500, Denver, Colorado 80202, with a request to forward the communication to the intended recipient or recipients. In general, any stockholder communication delivered to our Secretary for forwarding to the Board or specified Board member or members will be forwarded in accordance with the stockholder's instructions. However, our Secretary reserves the right not to forward to Board members any abusive, threatening or otherwise inappropriate materials. Information regarding the submission of comments or complaints relating to our accounting, internal accounting controls or auditing matters can be found on our website at www.coresite.com.

Table of Contents

Attendance of Directors at 20172018 Annual Meeting of Stockholders

        While we do not have a formal policy requiring our directors to attend stockholder meetings, directors are invited and encouraged to attend all meetings of stockholders. All of our directors at the time attended the 20172018 Annual Meeting of Stockholders.

Compensation Committee Interlocks and Insider Participation

        No member of the Compensation Committee is or has ever been an executive officer of our company, and no member of the Compensation Committee had any relationships requiring disclosure by us under the SEC's rules requiring disclosure of certain relationships and related-party transactions. None of our executive officers who have served as a director of our company or a member of the Compensation Committee have served on any compensation committee (or its equivalent) of any other entity.

Table of Contents

EXECUTIVE OFFICERS

        The following table sets forth certain information as of April 2, 2018,March 29, 2019, regarding our executive officers.

        Please see "Proposal One: Election of Directors—Directors – Directors" starting on page 5 for information regarding Mr. Szurek. There are no family relationships among our executive officers and directors.

        Jeffrey S. Finnin has served as our Chief Financial Officer since January 2011. Before joining us in January 2011, Mr. Finnin served as Managing Director and Chief Accounting Officer of Prologis, a publicly held REIT, for over five years. Mr. Finnin is responsible for our company's financial reporting, financial planning and analysis, treasury and investor relations activities. Prior to his tenure at Prologis, Mr. Finnin spent 18 years in public accounting, most recently as a partner with KPMG and Arthur Andersen LLP, where he served as the Industry Lead Partner in charge of Real Estate and Financial Services Practices in Denver, Colorado. Mr. Finnin received his B.S. in Business Administration from Colorado State University and is a Certified Public Accountant (inactive status).

        Derek S. McCandlessSteven J. Smith has served as our Senior Vice President, Legal, General CounselChief Revenue Officer since July 2018, and Secretary since March 2011. Prior to joining us in March 2011, Mr. McCandless served as Senior Vice President and Assistant General Counsel at Apartment Investment and Management Company, a REIT focused on apartment properties, which he joined in 2003. Prior to his tenure with Apartment Investment and Management Company, Mr. McCandless was in private practice with the law firms of Holme Roberts & Owen LLP (since combined with Bryan Cave LLP) and Cooley LLP. Mr. McCandless received a J.D. from The University of Chicago and a B.S., cum laude, from Brigham Young University.

Steven J. Smith has served as our Senior Vice President, Sales and Marketing sincefrom January 2014.2014 to July 2018. Mr. Smith is responsible for the success of our customer engagement, market expansion, sales engineering, product development, marketing and overall revenue growth. Prior to joining us in January 2014, Mr. Smith was a Regional Vice President for SAP AG, a business software company, from March 2008 to January 2014, where he led large enterprise sales across application portfolios, including enterprise resource planning, big data, customer relationship management, (CRM), cloud and mobile. From December 2002 to March 2008, he held various roles with Avaya, Inc., a global provider of business communications and collaboration systems, software and services, most recently as Vice President and General Manager of U.S. Channel Sales. Mr. Smith received a B.A. in Marketing and Economics from the University of New Mexico.

Dominic M. Tobin, who currently serves as Senior Vice President, Field Operations and Network Engineering, joined us in January 2007. Mr. Tobin is responsible for our company's field operations activities associated with our customer service and the maintenance, staffing, reliability and network engineering of our data centers. Mr. Tobin served as our Field Operations Director from 2007 to 2009 and Vice President of Operations from 2009 to 2010. Prior to joining us in January 2007, Mr. Tobin spent 22 combined years at First Level Technology and AT&T, where he held roles of increasing responsibility, including Field Operations Director and District Manager. Mr. Tobin obtained his B.S. in Telecommunications Management, magna cum laude, from Golden Gate University. He also received a Network Management Certificate from U.C. Santa Cruz Extension and was a First Class Electronics Technician in the U.S. Coast Guard.

Table of Contents

Brian P. Warren has served as our Senior Vice President, Engineering and Product since March 2014. Mr. Warren is responsible for our Company's product, facilities and sales engineering activities, including the design, construction, utilization and efficiency of our data centers. Mr. Warren joined us in October 2011 as our Vice President of Product Management and Market Development. Prior to joining us, he served as Senior Director of Internet Services Product Management at Level 3 Communications, a communications services company, where he held roles of increasing responsibility from June 2003 to October 2011. Prior to Level 3 Communications, Mr. Warren held various roles with Qwest Communications, @Link Networks, and Ernst &Young LLP. Mr. Warren began his career as a United Stated Air Force officer working with the North American Aerospace Defense Command (NORAD). Mr. Warren received an M.B.A. from Kellogg Graduate School of Management at Northwestern University and a B.S. in Engineering from the United States Air Force Academy.

        Juan A. Font has served as our Senior Vice President, General Management since March 2018. Mr. Font is responsible for maximizing revenue growth and return on invested capital across our portfolio. Mr. Font joined us in 2010 and has held positions of increasing responsibility, serving most recently as our Vice President of General Management. He brings over 15 years of experience in direct sales, business operations and finance in the data center and telecommunications industries. Prior to joining us, Mr. Font held direct sales contributor roles with Equinix, Inc., an owner and operator of data centers, covering strategic verticals. He also held roles of increasing responsibility in financial management and business operations across European markets with Teleglobe International Holdings Ltd., a provider of wholesale voice, data, IP and mobile signaling services. His previous experience also includes The World Bank Group, where he oversaw investments in the power sector. Mr. Font received a M.B.A. from The Kogod School of Business at The American University and a B.A. in Business Economics and Finance from the Universidad Complutense of Madrid.

Table of Contents

Maile C. Kaiser has served as our Senior Vice President, Sales since July 2018. She is responsible for overseeing revenue growth and managing the daily operations for our national sales organization. Ms. Kaiser joined us in 2012, serving previously as our Vice President of Sales for the Southwest and Central Regions, as well as Strategic Content Accounts. Ms. Kaiser brings over 20 years of direct sales and management experience in the datacenter, network communications, software and managed services industries driving growth and innovation for large enterprises. Prior to joining us, Ms. Kaiser was responsible for wholesale and large enterprise sales at IO Datacenters (now part of Iron Mountain Incorporated), a storage and information management service company, and ran the Southwest region for AboveNet Communications (now part of Zayo Group, LLC), a provider of high bandwidth connectivity solutions for businesses and carriers. Ms. Kaiser started her career with Oracle Corporation, a multinational computer technology corporation. Ms. Kaiser received a B.A. in English Literature from the University of California, at Berkeley.

Derek S. McCandless has served as our Senior Vice President, Legal, General Counsel and Secretary since March 2011. Prior to joining us in March 2011, Mr. McCandless served as Senior Vice President and Assistant General Counsel at Apartment Investment and Management Company, a REIT focused on apartment properties, which he joined in 2003. Prior to his tenure with Apartment Investment and Management Company, Mr. McCandless was in private practice with the law firms of Holme Roberts & Owen LLP (since combined with Bryan Cave Leighton Paisner LLP) and Cooley LLP. Mr. McCandless received a J.D. from The University of Chicago and a B.S., cum laude, from Brigham Young University.

Dominic M. Tobin, who currently serves as Senior Vice President, Field Operations and Network Engineering, joined us in January 2007. Mr. Tobin is responsible for our company's field operations activities associated with our customer service and the maintenance, staffing, reliability and network engineering of our data centers. Mr. Tobin served as our Field Operations Director from 2007 to 2009 and Vice President of Operations from 2009 to 2010. Prior to joining us in January 2007, Mr. Tobin spent 22 combined years at First Level Technology and AT&T, where he held roles of increasing responsibility, including Field Operations Director and District Manager. Mr. Tobin obtained his B.S. in Telecommunications Management, magna cum laude, from Golden Gate University. He also received a Network Management Certificate from U.C. Santa Cruz Extension and was a First Class Electronics Technician in the U.S. Coast Guard.

Brian P. Warren has served as our Senior Vice President, Development and Product Engineering since March 2014. Mr. Warren is responsible for data center and interconnection design, development and product engineering activities. Mr. Warren joined us in October 2011 and previously held positions leading our product, marketing and sales engineering activities. Prior to joining us, he served as Senior Director of Internet Services at Level 3 Communications, a communications services company, where he held roles of increasing responsibility from June 2003 to October 2011. Prior to Level 3 Communications, Mr. Warren held various roles with Qwest Communications, @Link Networks, and Ernst &Young LLP. Mr. Warren began his career as a United Stated Air Force officer working with the North American Aerospace Defense Command. Mr. Warren received an M.B.A. from Kellogg Graduate School of Management at Northwestern University and a B.S. in Engineering from the United States Air Force Academy.

Table of Contents

COMPENSATION DISCUSSION AND ANALYSIS

        This Compensation Discussion and Analysis is designed to provide our stockholders with a clear understanding of our compensation philosophy and objectives, compensation-setting process, and the 20172018 compensation of our named executive officers. For 2017,2018, our named executive officers were:

Executive Summary

        Our compensation program for our named executive officers and other executive officers is designed to meet the following primary objectives:

•

Management Development and Continuity.  Attract, retain and motivate individuals of superior ability and managerial talent to develop, grow and manage our business by offering competitive compensation opportunities with aboth an annual and significant long-term component;



•

Pay-for-Performance.  Align executive officer compensation with the achievement of our short- and long-term corporate strategies and business objectives and with the long-term interests of our stockholders through the use of performance-based and variable compensation elements; and



•

Long-Term Focus on Stockholder Value.  Align executives with stockholder value creation by delivering a significant portion of our executive officers' compensation in the form of equity-based awards that vest over multiple years.

        We believe compensation should be structured to ensure that a significant portion of the total compensation opportunity for our named executive officers is directly related to our performance and other factors that directly and indirectly influence stockholder value. The Compensation Committee focused on demonstrating its pay-for-performance philosophy and alignment of executive and stockholder interests in setting executive compensation by continuing to weight compensation toward performance-based pay. For 2017,2018, our fixed compensation versus targeted variable compensation was structured as follows for Mr. Szurek and the other named executive officers:

        Our 20172018 named executive officer compensation reflects the performance of our company in 2017.2018. Our 20172018 annual cash incentive program payouts were based on our above target performance in 20172018 revenue, EBITDAEBITDAre (as defined herein) and funds from operations (FFO)("FFO"). In addition, to further our commitment to a pay-for-performance compensation program, in March 2017,2018, the Compensation

Table of Contents

Committee granted performance-based restricted stock ("PSAs") to our senior leadership team, which vests based on the achievement of relative total shareholder return ("TSR") versus the MSCI US REIT Index over a three-year period.

Table of Contents

        We also believe our executive compensation should be structured to appropriately balance annual cash compensation with long-term equity-based compensation. For 2017,2018, our target annual cash versus long-term equity-based compensation was structured as follows for Mr. Szurek and the other named executive officers:

        In order to further align the long-term interests of management with those of our stockholders and align our compensation program with best practices, we (through our Board and Compensation Committee) establish and monitor specific policies, practices and processes.

Table of Contents

Highlights of Compensation Practices

Selected 20172018 Company Performance Highlights

        Performance for our company was strongsolid in 2017,2018, highlighted by the following:

Financial Performance in 2018:

•

Total shareholder return (assuming dividend reinvestment)Operating revenues grew in 2017 was 48.5%;2018 by 13.0% over 2017;



•

Revenue in 2017 grew by 20.3% over 2016;



•

Net income in 20172018 grew by 22.7% compared to 2016;



•

EBITDA (as defined below) in 2017 grew by 25.0% versus 2016;6.2% over 2017;

Table of Contents

•

EBITDAre in 2018 grew by 11.7% over 2017;



•

FFO (as defined below) attributable to common shares and units as adjusted, in 20172018 grew by 21.8%14.7%, or 12.4% on an adjusted basis (as defined herein), versus 2016;2017; and



•

Increased the quarterly common stock dividend by 22.5%in the fourth quarter to $0.98$1.10 per share, reflecting an annualannualized rate of $3.92$4.40 per share.share, an increase of 12.2% year over year. This represents the seventheighth consecutive year of double-digit growth in the quarterlyannualized fourth quarter dividend rate and 33% compound annual growth since our initial public offering.rate.

        In addition to the financial highlights above, we achieved the followingOperational Accomplishments in 2017:2018:

•

Achieved "Six"Seven 9s" uptime on all managed capacityreliability or "uptime" across our portfolio of data center facilities as we continued to focus on enhancing operational excellence;



•

Achieved strong organic growth, highlighted byincluding 81% of the 92% of 2017 leasing volume which cameannualized generally accepted accounting principles ("GAAP") rent from our new and expansion leases signed during 2018 from existing customers;



•

Commenced 136,902 NRSF174,834 net rentable square feet ("NRSF") of new and expansion leases representing $32.8$32.9 million of annualized GAAP rent at an average rate of $239 per square foot;



•

Completed development of 29,824 NRSF of turn-key data center capacity expansions at existing facilities across four markets, including Northern Virginia, Denver, Los Angeles and Boston;



•

Commenced the expansion of our Reston, Virginia data center campus with redevelopment and ground-up construction on two phases of turn-key data center capacity, cumulatively expected to add 64,922 square feet and 9 megawatts of power. The Reston expansion project is estimated to deliver 611,000 NRSF of incremental data center capacity across multiple phases with new buildings and as existing light-industrial / flex office leases expire and customers vacate. Based on our entitlement application, we believe we may be able to build an additional 286,000 NRSF for a total of 897,000 NRSF of incremental data center capacity;



•

Closed on our acquisition of a two-acre land parcel (referred to as SV8) immediately adjacent to our existing Santa Clara campus. We estimate we can build approximately 175,000 NRSF of turn-key data center capacity, or 18 megawatts of power capacity, across multiple phases;



•

Announced the expansion of our Los Angeles campus with the commencement of the entitlement, design, and permitting process for LA3, adjacent to LA2, to continue to meet customer demand in that market. We estimate we can build approximately 180,000 NRSF of turn-key data center capacity, or 18 megawatts of power capacity, across multiple phases;rent; and



•

Increased our customer base by 6%9% to more than 1,2001,350 of the world's leading enterprises, network operators, cloud providers, and supporting service providers.

Property Development Accomplishments in 2018:

•

Completed construction of approximately 172,000 NRSF, including:



o

Development of turn-key data center capacity expansions at our existing facilities including in Los Angeles, Denver and New York totaling 121,014 NRSF;



o

Construction of a new data center in our Northern Virginia market (referred to as DC2 in downtown Washington D.C.), including 24,563 NRSF and 1.5 megawatts of power;



o

Construction of the first phase of our Northern Virginia data center campus expansion (referred to as VA3, Phase 1A in Reston, Virginia), including 26,413 NRSF and three megawatts of power; and



o

Acquisition of a two-acre land parcel located in downtown Chicago (referred to as CH2).



•

Began construction of approximately 153,000 NRSF, including:



o

Development of turn-key data center capacity expansions at our two existing facilities in Los Angeles (referred to as LA1 and LA2) totaling 45,429 NRSF;



o

Development of our new ground up construction, including:



§

Our first phase of a new data center in the San Francisco Bay market (referred to as SV8 in Santa Clara, California) for 58,000 NRSF, and six megawatts of power; and



§

Our next phase at of turn-key data center capacity in Northern Virginia (referred to as VA3, Phase 1B, in Reston, Virginia) for 49,837 NRSF, and six megawatts of power.

Table of Contents

•

Began pre-construction of approximately 118,000 NRSF, including:



o

The first phase of a new data center in downtown Chicago (referred to as CH2) of 58,000 NRSF, and six megawatts of power; and



o

The first phase of a new data center in Los Angeles (referred to as LA3) of 60,000 NRSF, and six megawatts of power.



•

Other development and activities, including:



o

Ongoing strategic development to expand our Santa Clara, California campus that culminated in a contract in January 2019 to acquire a 3.8-acre operating light-industrial / flex office building and land parcel located adjacent to our existing campus; and



o

Closed on our acquisition of U.S. Colo, a carrier-neutral, network-dense colocation provider in Los Angeles, for a total of 28,573 NRSF.

        See Appendix A to this proxy statement for a reconciliation of EBITDAEBITDAre and FFO to the nearest GAAP financial measure.

        At our annual meeting of stockholders in May 2017,2018, we held our annual advisory vote to approve the compensation of our named executive officers ("say-on-pay"). The compensation of our named executive officers reported in our 20172018 proxy statement was approved by 96.8%97.0% of the votes cast at the 2017 annual meeting.2018 Annual Meeting of Stockholders. Our Compensation Committee believes this affirms our stockholders' support of our approach to executive compensation.compensation, and, as a result, the Compensation Committee did not make any significant changes to our executive compensation program for 2018. The Compensation Committee will continue to consider the outcome of our say-on-pay votes when making future compensation decisions for our named executive officers.

Role of the Board of Directors, the Compensation Committee, Management and Consultant

        Our Compensation Committee is charged with, among other things, the responsibility of reviewing executive officer compensation policies and practices to ensure adherence to our compensation

Table of Contents

philosophy and objectives and that the total compensation paid to our executive officers is consistent with our performance, fair, reasonable and competitive with companies within our industry. The Compensation Committee's primary responsibilities with respect to determining executive compensation are (i) setting performance targets under all annual bonus and long-term and management incentive compensation plans, including our equity compensation plans; (ii) verifying that performance targets used for any performance-based equity compensation plans have been met before payment of any executive bonus or compensation; (iii) approving all amendments to, and terminations of, all compensation plans and any awards under such plans; (iv) granting any awards under any performance-based annual bonus, long-term incentive compensation and equity compensation plans to executive officers; (v) approving which executive officers and other employees receive awards under our equity and incentive compensation plan(s), including our equity compensation plans; and (vi) conducting an annual review of all compensation plans. In some cases, the Compensation Committee makes recommendations that our Board approve these items or grant equity awards. All plan reviews include reviewing the plan's administrative costs, reviewing current plan features relative to any proposed new features, and assessing the performance of the plan's internal and external administrators if any duties have been delegated.

        The Compensation Committee reviews and considers our Chief Executive Officer's recommendations with respect to compensation decisions for our named executive officers, other than himself. The Compensation Committee believes it is valuable to consider the recommendations of our

Table of Contents

Chief Executive Officer with respect to these matters because, given his knowledge of our operations, the data center industry and the day-to-day responsibilities of our executive officers, he is in a unique position to provide the Compensation Committee perspective into the performance of our executive officers in light of our business at a given point in time. The Board (without the participation of our Chief Executive Officer) and Compensation Committee makes all compensation decisions with regard to our Chief Executive Officer.

        As part of the 20172018 compensation process, the Compensation Committee retained W.T. Haigh & Company, Inc. ("W.T. Haigh") as its independent compensation consultant. W.T. Haigh provides us advisory services only with respect to executive compensation, and works with management only with the approval and under the direction of the Compensation Committee. W.T. Haigh reviewed the compensation components of our 20172018 program for our named executive officers and advised the Compensation Committee regarding the components and levels of the executive compensation program, including our incentive and equity-based compensation plans. W.T. Haigh also assists the Board in its annual Board and committee evaluation process. A representative of W.T. Haigh attended all of the Compensation Committee meetings in 20172018 and to date, all of the Compensation Committee meetings in 2018.2019. W.T. Haigh continues to make itself available on an ongoing basis to provide guidance to the Compensation Committee on compensation issues as they arise. The Compensation Committee has reviewed its and our company's relationships with W.T. Haigh and has not identified any conflicts of interest.

Peer Companies

        The Compensation Committee uses peer company data to guide its review of the total compensation of our executive officers and generally reviews the compensation data of our peer companies and industry to understand market competitive compensation. The Compensation Committee focuses on ensuring that the elements of our executive compensation program are consistent with peer and industry trends. The Compensation Committee generally targets total compensation to between the market 50th and 75th percentile consistent with our company's performance, with top performers targeted toward the top end of the range. However, the Compensation Committee does not benchmark compensation to a specific percentile target or range of the compensation of peer companies.

        For purposes of the 20172018 compensation decisions, the Compensation Committee approved a peer group based on analysis and recommendations by W.T. Haigh. This peer group is comprised of 1413 companies in the data center, special purpose real estate, hosting and colocation industries. The peer

Table of Contents

group was selected based primarily on similarity to our core business as well as a number of other criteria, including each company's revenue, market capitalization, number of employees and other key financial metrics. We believe these companies are broadly comparable to us, and appropriately reflect our size, complexity, growth prospects, operations, and the labor market for key leadership positions. The 20172018 peer companies are listed below:

Table of Contents

        For 2017,2018, the Compensation Committee added Corporate Office Properties Trust and Zayo Group HoldingsSwitch, Inc. to the peer group due to theirits focus on data center ownership and operations and theirits comparable market capitalization measures relative to CoreSite. With respect to Corporate Office Properties Trust, the Compensation Committee also considered its geographic focus in the greater Washington, D.C. area. Rackspace Hosting,DuPont Fabros Technology, Inc. was removed from the peer group as it was acquired by Digital Realty Trust, Inc. in 2016.the third quarter of 2017. Iron Mountain Incorporated was also removed from the peer group as its revenues from data center operations constitute a relatively small percentage of its total revenues. In addition to the select peer companies above, compensation data was used that covers substantially all of the companies in the MSCI US REIT Index and the NASDAQ Internet Index.

        The Compensation Committee reviews our peer group annually to determine whether changes are necessary to ensure that the peer companies continue to be comparable to us based on changing scope and growth characteristics.

Elements of 20172018 Compensation

        Our compensation program is made up of the following direct compensation elements:

        For 2017,2018, the following was the targeted mix of compensation:

Table of Contents

20172018 Compensation Decisions

        Base Salaries. In early 2017,2018, the Compensation Committee reviewed base salaries to ensure that they generally were competitive with market levels and generally reflected our level of financial performance during the previous year. No formulaic base salary increases are provided to our named executive officers; however, annual merit increases are provided when the Compensation Committee determines that such increases are warranted in light of national salary increase levels, salary levels within our peer companies, individual performance and/or overall company performance.

Table of Contents

        The base salaries of our named executive officers in 20172018 were as follows:

        The base salaries for each of our named executive officers set forth above were effective as of April 1, 2017.March 26, 2018. For each of our named executive officers, (except Mr. Szurek), salary increases in 20172018 were driven in large part by each named executive officer's relative position to mid-market salary levels when compared to the peer companies and broader market comparisons, along with individual performance evaluations. The annualIn particular, the base salary for Mr. Szurek was set in September 2016 in connectionadjusted to make it more competitive with the negotiation of his employment agreement and was influenced by aour peer company comparative review from W.T. Haigh and efforts to induce Mr. Szurek to join us as our President and Chief Executive Officer.companies.

        Annual Cash Incentive Awards. An important component of our total compensation program is the annual cash incentive based on the achievement of preset, annual company performance objectives and individual executive performance, which is determined in the Compensation Committee's discretion.

        The Compensation Committee established the following 20172018 target annual incentive amounts for each of our named executive officers under our 20172018 annual cash incentive program (the "2017"2018 Bonus Plan"):

        In setting the 20172018 target bonus amounts for each of our named executive officers, the Compensation Committee considered the following factors: (i) organizational level and expected impact on our annual operating results; (ii) the scope, level of expertise and experience required for the named executive officer's position; and (iii) competitive levels of target annual incentive opportunity. The 2017Based on the forgoing factors, the Compensation Committee determined to keep the 2018 target bonus percentages for Messrs. Szurek, Finnin and McCandless were increased as a resulteach of their individual performance and in an effort to better align their annual cash incentive awardsthe named executive officers consistent with those of our peer companies. The 2017the target bonus percentages for Mr. Warren was not adjusted from 2016, and the 2017 target bonus percentage for Mr. Smith was decreased slightly due to the

Table of Contents

increase in his base salary in 2017. Participants under the 20172018 Bonus Plan were eligible to receive between 0% and 175% of each participant's respective target bonus based on actual performance as discussed below.

        Actual bonus amounts earned for 20172018 were based on the level of company-wide achievement of revenue, EBITDAEBITDAre and FFO in 20172018 versus targets established by the Compensation Committee at the beginning of the year. The total incentive bonus actually paid to each named executive officer is determined based on the extent to which specified weighted objective company performance goals are achieved, multiplied by a weighting and bonus factor. The bonus factors for each performance factor

Table of Contents

used to calculate the total incentive bonuses as approved by the Compensation Committee were as follows and the actual(actual results within the range in bold/italics:italics):

        The Compensation Committee or the Board has the discretion to adjust the target or actual results based on extraordinary events and/or conditions that either positively or negatively impact our performance. For purposes of the 20172018 Bonus Plan, target FFO for 20172018 was increaseddecreased from the number set at the beginning of the year to account for the issuance of debt in the second quarter of 2017.2018 and variances to the LIBOR rate on our outstanding debt. In addition, the Compensation Committee or the Board may further adjust the bonus payments in its discretion based on each named executive officer's achievements and overall job performance

Table of Contents

during 2017.2018. The table below sets forth the calculation of the 20172018 bonus payouts with respect to the objective company performance goals:

        Funds

Table of Contents

        The Compensation Committee also adjusts the bonus payout based on the individual performance of these measures to GAAP.

each executive officer. In light of our performance in 20172018 and the Board's discretionary evaluation of each named executive officer's job performance, with input from our Chief Executive Officer (except with respect to his own compensation) and upon recommendation from the Compensation Committee,, we paid the following annual bonus amounts under the 20172018 Bonus Plan:

        In determining the increased bonus amounts for Messrs. Szurek, Finnin and Smith, the Board considered the individual performance evaluations for each person in addition to overall Company performance in 2017.

        Equity Compensation. Generally in the first quarter of each year, the Board or Compensation Committee grants equity-based awards to our named executive officers, other executives, key management personnel and other employees in order to align their interests with those of the stockholders and to provide a compensation element intended to retain our named executive officers and other executives over the long term. For 2017,2018, the value of the long-term equity awards granted to each named executive officer was based on the Board's and Compensation Committee's assessment of each named executive officer's expected future contributions to our company, ability to impact our long-term results that drive stockholder value, each named executive officer's overall long-term performance and competitive levels of long-term equity compensation for similarly situated executives at our peer companies.

        In 2017,2018, the Compensation Committee and the Board approved equity awards to our senior leadership team, including our named executive officers, with 60% of the value of the equity award in the form of time-based restricted stock and 40% of the value of the equity award in the form of PSAs (except for Mr. Szurek). For 2018, the weighting of performance-based restricted stock ("PSAs"). The Compensation Committee believesfor Mr. Szurek was increased to 60%, with the weighting of time-based restricted stock reduced to 40%, to further strengthen our pay-for-performance philosophy and alignment with shareholders' interests. We believe that time-based restricted stock awards offer a strong retention incentive to employees and are less dilutive to our current stockholders than stock options. The time-based restricted stock awards vest in three equal annual installments beginning on the first anniversary of the grant date, subject to continued employment with us.

Table of Contents

        The Compensation CommitteeWe began granting PSAs in 2014 and continued its practice of granting PSAs in 20172018 to further our objective of a pay-for-performance compensation program to tie executive compensation to the achievement of our longer-term corporate strategies and business objectives and to the long-term interests of our stockholders. The Compensation CommitteeWe determined to grant the PSAs in the form of restricted stock to place greater emphasis on our performance relative to other REITs. In 2017,2018, PSAs were granted only to our senior leadership team, including our named executive officers, while other employees of our company received equity awards solely in the form of time-based restricted stock. The PSAs granted in 20172018 (the "2017"2018 PSAs") vest in full on the third anniversary of the grant based on our achievement of relative total shareholder return ("TSR")TSR measured versus the MSCI US REIT Index over a three-year performance period. The number of shares that may be earned under the 20172018 PSA grants ranges from 25% to 175% of target, as described further below.

Table of Contents

        The 20172018 PSAs are earned as follows: (i) 20% of the PSAs are earned based on relative TSR achievement for each annual performance period during the three-year performance period (60% in total), and (ii) 40% of the PSAs are earned based upon a cumulative TSR achievement over the three-year performance period. Earned PSAs are released at the end of the three-year performance period, provided that the executive officer continues to be employed by us at the end of the performance period. Holders of earned PSAs are also entitled to accrued cash dividends that will be paid at the end of the performance period. The table below sets forth the performance multipliers for each level of TSR achievement relative to the MSCI US REIT Index to be used for each vesting period of the 20172018 PSAs:

        The PSA grants developed and implemented for 20172018 were based on analysis of peer company and broader market performance-based equity practices, as prepared by W.T. Haigh, and are consistent with our performance-based compensation philosophy.

        The equity awards granted to our named executive officers in 20172018 were as follows:

PSA Performance to Date

        In March 2018,2019, the Board and the Compensation Committee certified TSR achievement for 2017 of 175% for the 2017, 2016 and 2015 PSAs, which resulted in earning the maximum payout for the

Table of Contents

20172018 performance period, based on our relative performance versus the MSCI US REIT Index, at below the 100th percentile.20th percentile, which resulted in earning 25% of the target PSAs, or a weighted payout of 5% of the target PSAs for the 2018 performance period. For the 20152016 PSAs, this also resulted in anthe overall payout for the cumulative three-year period was at the maximum amount of 175% of the target sharesshares. The final total earned award for the three-year period.2016 PSAs was 145% of the target shares.

Table of Contents

        Performance under the performance cycles to date for the 20152016 PSAs, the 20162017 PSAs and the 20172018 PSAs are as follows:

2015 PSA Performance Cycle

2016 PSA Performance Cycle

2017 PSA Performance Cycle

2018 PSA Performance Cycle

Defined Contribution Plans. We have maintained a Section 401(k) Savings/Retirement Plan (the "401(k) Plan") for eligible employees of our company and any designated affiliate, including our named executive officers. The 401(k) Plan provides our named executive officers and other employees with the opportunity to save for their future retirement by deferring compensation up to IRS imposed limits. We currently makeIn 2018, we made safe harbor contributions to the 401(k) Plan in an amount equal to three percent (3%) of the participant's annual salary and subject to certain other limits.limits, which safe harbor contribution amounts will increase to four percent (4%) in 2019. Plan participants vest immediately in the amounts contributed by us. Our employees are eligible to participate in the 401(k) Plan following their first month of full employment, with safe harbor contributions beginning after six months of credited service.

Other Elements of Compensation and Perquisites. In addition to other elements of compensation, as described above, we provide the following other compensation and benefits to our named executive officers:

•

Medical Insurance.Medical.  We offer to each named executive officer, the named executive officer's spouse and the named executive officer's children such health, dental and vision insurance programs as we make available to other eligible employees of our company. Executive

Table of Contents

officers are also eligible for annual executive physical examinations beyond those covered by our general health insurance programs.

•

Life and Disability Insurance.  We provide each named executive officer such short-term and long-term disability and/or life insurance as we make available to other eligible employees of our company. Our company offers life insurance coverage equal to the annual salary of each employee, up to a designated maximum amount per employee.



•

Parking Allowance.  We provide each named executive officer with the choice of paid parking at each company location or reimbursement of public transportation expenses, such as our company makes available to every other employee of our company.



•

Relocation.  In 2016 and 2017, we provided relocation assistance to Mr. Szurek to Denver, Colorado in connection with his appointment as our President and Chief Executive Officer.

Other Compensation Components

        We believe that it is important to maintain flexibility to adapt our compensation structure to properly attract, motivate, and retain the top executive talent for which we compete. We may provide compensation components that are different from or in addition to the components described above to our named executive officers, to ensure that we provide a balanced, comprehensive and competitive compensation structure, as deemed appropriate by the Compensation Committee.

Accounting Considerations

        ASC Topic 718,Compensation—Stock Compensation (referred to as ASC Topic 718), requires us to recognize an expense for the fair value of equity-based compensation awards. Grants of stock options, restricted stock, restricted stock unitsRSUs and performance units under our equity incentive award plans will be accounted for under ASC Topic 718. We will consider the accounting implications of significant compensation decisions, especially in connection with decisions that relate to our equity incentive award plans and programs. As accounting standards change, we may revise certain programs to appropriately align accounting expenses of our equity awards with our overall executive compensation philosophy and objectives.

Compensation Risk Assessment

        As discussed in more detail above, the Compensation Committee has reviewed our compensation programs for all employees, including our named executive officers, and other executives and has concluded that these programs do not create risks that are reasonably likely to have a material adverse effect on our company.

Table of Contents

COMPENSATION COMMITTEE REPORT

        The Compensation Committee (the "Compensation Committee") of the Board of Directors (the "Board") of CoreSite Realty Corporation reviewed and discussed the Compensation Discussion and Analysis included in this proxy statement with management. Based on such review and discussion, the Compensation Committee recommended to the Board that the Compensation Discussion and Analysis be included in this proxy statement for filing with the Securities and Exchange Commission.

The Compensation Committee:

Michael R. Koehler (Chair)
Kelly C. Chambliss
J. David Thompson
David A. Wilson

Table of Contents

20172018 Summary Compensation Table

        The following table sets forth certain information with respect to the compensation paid to our named executive officers during the fiscal years ended December 31, 2018, 2017 2016 and 2015,2016, as applicable. Unless otherwise specified, positions listed below are those currently held by the named executive officers.