SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 15)*

CommonWealth REIT

(Name of Issuer)

Common Shares of Beneficial Interest, par value $0.01 per share

(Title of Class of Securities)

203233101

(CUSIP Number)

Keith Meister

Corvex Management LP

712 Fifth Avenue, 23rd Floor

New York, New York 10019

(212) 474-6700

Richard O’Toole

Related Fund Management, LLC

60 Columbus Circle

New York, New York 10023

(212) 421-5333

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

February 11, 2014

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box  ¨.

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

This Amendment No. 15 to the Schedule 13D (this “Amendment No. 15”) relates to the common shares of beneficial interest, par value $0.01 per share (the “Shares”), of CommonWealth REIT, a Maryland real estate investment trust (the “Issuer” or “CommonWealth”) and amends the Schedule 13D filed on February 26, 2013, as amended by Amendment No. 1 thereto, filed with the SEC on February 27, 2013, Amendment No. 2 thereto, filed with the SEC on March 4, 2013, Amendment No. 3 thereto, filed with the SEC on March 4, 2013, Amendment No. 4 thereto, filed with the SEC on March 11, 2013, Amendment No. 5 thereto, filed with the SEC on March 13, 2013, Amendment No. 6 thereto, filed with the SEC on March 15, 2013, Amendment No. 7 thereto, filed with the SEC on March 28, 2013, Amendment No. 8 thereto, filed with the SEC on April 12, 2013, Amendment No. 9 thereto, filed with the SEC on April 18, 2013, Amendment No. 10 thereto, filed with the SEC on June 20, 2013, Amendment No. 11 thereto, filed with the SEC on June 24, 2013, Amendment No. 12 thereto, filed with the SEC on August 8, 2013, Amendment No. 13 thereto, filed with the SEC on November 19, 2013 and Amendment No. 14 thereto, filed with the SEC on November 25, 2013 (the “Original Schedule 13D” and, together with this Amendment No. 15, the “Schedule 13D”). Capitalized terms used and not defined in this Amendment No. 15 have the meanings set forth in the Original Schedule 13D.

This Amendment No. 15 is being filed by (i) Corvex Management LP, a Delaware limited partnership (“Corvex”), and Keith Meister, (ii) Related Fund Management, LLC, a Delaware limited liability company (“Related”), Related Real Estate Recovery Fund GP-A, LLC, a Delaware limited liability company, Related Real Estate Recovery Fund GP, L.P., a Delaware limited partnership, Related Real Estate Recovery Fund, L.P., a Delaware limited partnership (“Related Recovery Fund”), and RRERF Acquisition, LLC, a Delaware limited liability company and (iii) David R. Johnson (the “Individual Shareholder”).

This Amendment No. 15 is being filed to amend Item 4, Item 6 and Item 7 of the Schedule 13D as follows:

Item 4 of the Schedule 13D is amended by adding thereto the following:

On February 11, 2014 Corvex and Related Recovery Fund entered into an agreement (the “EGI Agreement”) with EGI-CW Holdings, L.L.C. (“EGI-CW”), a Delaware limited liability company and a wholly-owned subsidiary of EGI Fund (11-13) Investors, L.L.C., an Equity Group Investments company (“EGI”), which is a 40+ year old, Chicago-based private investment firm founded by Samuel Zell.

Expanded Slate

Pursuant to the EGI Agreement, the slate of Nominees (each a “Nominee” and together, the “Slate”) proposed by certain of the Reporting Persons has been increased from five to seven to add: Samuel Zell, Chairman of EGI, and David Helfand, Co-President of EGI. Each of Mr. Zell and Mr. Helfand has also entered into a nominee agreement (each, a “Nominee Agreement” and, collectively the “Z/H Nominee Agreements”) pursuant to which they have agreed to serve as trustees of the Issuer if elected. In addition, Mr. Zell and Mr. Helfand have agreed that if the Slate is elected to constitute a majority of the Board of Trustees of the Issuer (the “Board”), they are willing to serve as Chairman of the Board and Chief Executive Officer of the Issuer, respectively, if so appointed by the new Board of Trustees, subject to good faith negotiation of mutually acceptable employment and compensation arrangements between each of them and the Issuer. Pursuant to the EGI Agreement, Corvex, Related Recovery Fund and EGI-CW have agreed to only nominate a slate that is publicly committed to effecting the foregoing appointments, subject to their fiduciary duties as trustees.

Options

Pursuant to the EGI Agreement, (A) Corvex has granted to EGI-CW an option (the “Corvex Option”) to purchase from Corvex (i) up to 1,190,476 Shares at a price per Share of $21 and (ii) up to 833,333 Shares at a price per Share of $24; and (B) Related Recovery Fund has granted to EGI-CW an option (the “Related Option” and, together with the Corvex Option, the “Options”) to purchase from Related Recovery Fund (i) up to 1,190,476 Shares at a price per Share of $21 and (ii) up to 833,333 Shares at a price per Share of $24. Each of the Options may be exercised by EGI-CW in whole or in part, at any time and from time to time, (i) commencing on the date that is the earliest of (w) the date immediately following the date of the approval of the proposal to remove all of the Trustees of the Issuer (the “Removal Proposal”) by the Issuer’s shareholders in the related consent solicitation, (x) the date that is eight business days prior to the announced or anticipated expiration of any Qualified Tender Offer or the announced or anticipated closing of any Qualified Sale Transaction, (y) the date that is five business days prior to the date on which Corvex or Related Recovery Fund propose to sell, dispose of or otherwise transfer more than 5% of the Shares it beneficially owns as of the date of the EGI Agreement to a person or entity that is not an affiliate thereof (provided that Corvex and Related Recovery Fund shall give EGI-CW written notice on such fifth business day prior to such sale, disposition or transfer), and (z) the date on which Corvex and Related Recovery Fund first publicly announce that they do not intend to continue to pursue the replacement of the current Board, or if earlier, the date on which they cease all meaningful efforts to replace the current Board, and (ii) continuing until the 60th calendar day following the date on which the Slate, including Mr. Zell and Mr. Helfand, is duly elected by the shareholders of Issuer, Mr. Zell is duly elected as Chairman of the Board of the Issuer and Mr. Helfand is duly appointed Chief Executive Officer of the Issuer (collectively, the “New Board Election”) and all court, arbitral and other challenges to such elections and appointment have been finally and favorably resolved in favor of the Slate, Mr. Zell and Mr. Helfand, respectively (the “Option Period”). The Options will terminate at the earliest of (i) the expiration of the Option Period and (ii) at such time as (x) either of the Z/H Nominee Agreements is terminated pursuant to its terms or (y) a Qualified Tender Offer or Qualified Sale Transaction is consummated. “Qualified Tender Offer” means any tender or exchange offer by any person for at least a majority of the outstanding Shares. “Qualified Sale Transaction” means any merger or consolidation transaction involving the Issuer.

Voting Undertaking

Pursuant to the EGI Agreement, each of Corvex and Related Recovery Fund have agreed to, within one business day following the New Board Election, deliver to the new Board of Trustees of the Issuer an undertaking that provides that for a period of one year following the date of the New Board Election, each of Corvex and Related Recovery Fund will cause with respect to Shares they or any of their Covered Entities beneficially own, (a) as of the record date for each meeting of shareholders of the Issuer, to be present for quorum purposes and to be voted at each such shareholder meeting or at any adjournments or postponements thereof, in favor of all actions recommended by the Board in its definitive proxy statement for such shareholder meeting; and (b) as of the record date for any proposed action by written consent, to consent to all actions recommended by the Board in its definitive consent solicitation statement and to take no action with respect to shareholder proposals not recommended by the Board. “Covered Entities” with respect to any party, means such party’s controlled affiliates, and any of such party’s and such party’s controlled affiliates’ respective investment funds, managed accounts or other investment vehicles managed or advised by them, provided, however, that in no event shall any person be deemed to be a Covered Entity solely because another person serves as a director, trustee or officer of such person.

Purchase of Securities

Pursuant to the EGI Agreement, and except for shares beneficially owned by Corvex and Related Recovery Fund as of the date of the EGI Agreement, neither Corvex, Related Recovery Fund or EGI-CW shall, directly or indirectly, including, without limitation, through any of their Covered Entities, and each of Corvex, Related Recovery Fund and EGI-CW shall cause its affiliates not to, (i) beneficially own, purchase or otherwise hold or acquire, or propose or offer to purchase or acquire, any Company Securities or any interests or rights in respect of any Company Securities, or (ii) enter into any agreement, arrangement or understanding with any person (other than an affiliate) in connection with the holding, voting or disposition of Company Securities. Notwithstanding the foregoing, Corvex and Related Recovery Fund (together with their Covered Entities) may purchase or otherwise acquire common shares, so long as the total aggregate number of Shares they beneficially own is less than 9.8% of the total number of outstanding Shares. “Company Securities” means, collectively, the Shares and any other equity or debt securities of the Issuer, or other securities convertible into or exchange for equity or debt securities of the Issuer.

Indemnification

Pursuant to the Z/H Nominee Agreements and the EGI Agreement, Corvex and Related Recovery Fund will, among other things, indemnify EGI-CW, Mr. Zell and Mr. Helfand against certain potential liabilities. Such indemnification provisions only cover Mr. Zell and Mr. Helfand’s service as a Nominee and not, if elected, as a trustee of the Issuer.

Termination

Each of the Z/H Nominee Agreements shall terminate upon the earliest of (i) 11:59 p.m. (New York City time) on May 8, 2016, (ii) such earlier time as may be agreed to in writing by the Nominee, Corvex and Related Recovery Fund and (iii) the delivery by the Nominee of written notice to Corvex and Related Recovery Fund at any time prior to his election to the Board (such notice, a “Withdrawal

Notice”), provided, that neither Mr. Zell nor Mr. Helfand may deliver a Withdrawal Notice or otherwise terminate the Z/H Nominee Agreements under clause (iii) either (A) prior to August 10, 2014 or (B) after any of the Options granted pursuant to the EGI Agreement have been exercised, either in whole or in part.

The EGI Agreement will terminate upon the earliest of (i) 11:59 p.m. (New York time) on May 8, 2016, (ii) the delivery of written notice by EGI-CW to Corvex and Related Recovery Fund at any time prior to the New Board Election (provided that such notice may not be delivered, and the EGI Agreement may not be terminated pursuant to clause (ii) either (A) prior to August 10, 2014 or (B) after any of the Options have been exercised, either in whole or in part), (iii) one business day following the New Board Election (provided that the Options will survive termination pursuant to clause (iii) and continue in effect in accordance with their terms), (iv) termination of either of the Nominee Agreements in accordance with the terms thereof, and (v) such earlier time as may be agreed to in writing by the parties, provided that the indemnification obligations will survive any termination of the EGI Agreement.

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The foregoing summary of the EGI Agreement does not purport to be complete and is subject to, and qualified in its entirety by, the full text of the EGI Agreement, which is filed as Exhibit 32 hereto, and is incorporated by reference in this Item 4 in its entirety.

On February 11, 2014, Corvex and Related issued a press release. The press release is attached as Exhibit 33 hereto and is incorporated by reference in this Item 4 in its entirety.

Item 6 of the Schedule 13D is amended by adding thereto the following:

The information set forth under Item 4 of this Amendment No. 15 is incorporated in this Item 6 in its entirety.

Item 7 of the Schedule 13D is amended by adding thereto the following:

SIGNATURES

After reasonable inquiry and to the best of our knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

EXHIBIT 32

AGREEMENT

THIS AGREEMENT, dated as of February 11, 2014 (this “Agreement”), is by and among Corvex Management LP (“Corvex”), Related Real Estate Recovery Fund, L.P. (“Related”) and EGI-CW Holdings, L.L.C., a Delaware limited liability company (“EGI”). Corvex, Related and EGI each are referred to herein as a “Party”, and collectively as the “Parties.”

WHEREAS, on January 29, 2013, Corvex and Related entered into an agreement (the “Corvex/Related Agreement”) to coordinate certain efforts with respect to (i) the proposal of certain actions relating to CommonWealth REIT (“CommonWealth” or the “Company”) and (ii) the purchase of common shares of beneficial interest of the Company (“Shares”) by them or their Covered Entities (as defined below);

WHEREAS, on November 18, 2013, an arbitral panel issued an interim arbitration award which, among other things, set forth a process for a consent solicitation for the removal of Trustees of the Company and, if such consent solicitation is successful, the holding of a special meeting of shareholders of the Company to elect new Trustees;

WHEREAS, Corvex and Related have commenced the process to solicit from CommonWealth shareholders consents to remove the entire Board of Trustees of the Company (the “Board”) (the “Removal Proposal”);

WHEREAS, if, as a result of the Removal Proposal, all of the members of the Board of Trustees of the Company are removed, Corvex and Related intend to nominate and solicit proxies for the election of a slate (“Slate”) of new Trustees at a special meeting of shareholders that must be promptly called in the event that the Removal Proposal is successful (the “Election Proposal” and, together with the Removal Proposal, the “Proposals”); and

WHEREAS, concurrently with the execution hereof Samuel Zell (“Z”) and David Helfand (“H”) have each delivered to Corvex and Related a letter agreement (each, a “Nominee Agreement” and, collectively, the “Nominee Agreements”).

NOW, THEREFORE, in consideration of the covenants and agreements set forth in this Agreement, and for other good and valuable consideration the receipt and sufficiency of which are acknowledged, and intending to be legally bound, the Parties agree as follows:

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3

4

5

(c) The indemnification of Z and H provided for in this Section 8 shall be separate from, and in addition to, the indemnification rights of Z and H under their respective Nominee Agreements.

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[Signature Page Next]

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The Parties have caused this Agreement to be executed as of the day and year first above written.

[Signature Page to EGI Agreement]

[Signature Page to EGI Agreement]

Schedule I

EXHIBIT 33

SAM ZELL AND DAVID HELFAND JOIN CORVEX AND RELATED

BOARD SLATE OF HIGHLY QUALIFIED NOMINEES FOR

COMMONWEALTH REIT

Support Consent Solicitation for Removal of Entire Board of Trustees to

Unlock Value for All CommonWealth Shareholders

New York – February 11, 2014 – Corvex Management LP and Related Fund Management, LLC today announced that Sam Zell and David Helfand, veteran REIT executives, are joining the slate of highly qualified, independent nominees for election to the Board of Trustees of CommonWealth REIT (NYSE: CWH) if the pending consent solicitation to remove all the current Trustees of CommonWealth REIT is successful. Since February 2013, Corvex and Related have been engaged in an effort to remove CommonWealth’s entire Board of Trustees in order to unlock substantial value for all shareholders. Separately managed investment funds of Related and Corvex own approximately 9.6 % of the outstanding shares of CommonWealth.

Sam Zell commented, “We are fully supportive of Corvex and Related’s efforts to maximize value at CommonWealth for all shareholders. We see an attractive opportunity at CommonWealth uniquely suited to our expertise in leading public real estate companies and in turning around underperforming assets. We created three of the most successful REITs in the U.S., including Equity Office, which at the time of its $39 billion sale in 2007, owned nearly 100 million square feet of space in over 500 office buildings across the country.

“One of our core operating principals is the alignment of interests between company leadership and shareholders,” he continued. “We are concerned about any attempts to preclude shareholder rights, and our companies are free of such impediments. We believe the shareholders of our REITs have clearly benefited from having an accountable, properly aligned board overseeing an effective, internalized management team with the sole goal of increasing shareholder value.”

Zell and Helfand will join the proposed slate of directors to be nominated at a special meeting of CommonWealth shareholders if the current Board of Trustees is successfully removed. The other five nominees are James Corl, Edward Glickman, Peter Linneman, Kenneth Shea and Jim Lozier, who will head up transition efforts. In addition, Zell and Helfand have agreed that if the Corvex/Related slate is elected to constitute a majority of the Board, they are willing to serve as Chairman of the Board and Chief Executive Officer of CommonWealth, respectively, if so appointed by the new Board of Trustees, subject to their fiduciary duties. In addition, Zell and Helfand plan to bring to the company their highly qualified and experienced management team to execute on a value-driven strategy.

Keith Meister of Corvex and Jeff T. Blau of Related said, “We are thrilled to be joined by Sam Zell, whose chairmanship of other REITs has unquestionably maximized value for shareholders. Additionally, David Helfand’s unmatched real estate management skills and superb track record in executive leadership qualify him to enhance CommonWealth’s portfolio. We believe that once CommonWealth shareholders are given a choice between the Portnoys and their record of value destruction and Sam Zell’s record of value creation for shareholders, the choice is clear.”

Neither Zell, nor Helfand or their affiliates owns any CommonWealth stock. Corvex and Related have granted an entity associated with Zell and Helfand an option to acquire up to approximately 4 million CommonWealth shares, subject to various terms and conditions.

Zell maintains substantial interests in and serves as Chairman of four public companies, two of which are REITs — Equity Residential (NYSE: EQR), the largest multifamily REIT, and Equity LifeStyle Properties (NYSE: ELS), the largest manufactured home community REIT. He is also Chairman of Covanta Holding Corporation (NYSE: CVA), an international leader in converting waste to energy, and Anixter International (NYSE: AXE), a global supplier of communications and security products. Zell also serves as Chairman of two private investment firms, Equity Group Investments (EGI), which he founded over 40 years ago, and Equity International. While EGI’s roots are in real estate, the firm’s investments today span industries and continents, and include interests in real estate, energy, logistics, transportation, media, and health care, among others. Equity International, which Zell founded in 1999, is a private investment firm focused on building real estate-related businesses in international emerging markets. Zell is a member of the President’s Advisory Board at the University of Michigan, and with the combined efforts of the University of Michigan Business School, established the Zell/Lurie Entrepreneurial Center. He is also a long-standing supporter of the University of Pennsylvania Wharton Real Estate Center, and has endowed the Samuel Zell/Robert Lurie Real Estate Center at Wharton. Zell also endowed the Northwestern University Center for Risk Management. Zell holds a BA and a JD from the University of Michigan.

Helfand is currently Co-President of EGI where he oversees all aspects of the firm. He began working with Zell more than 25 years ago, and has worked with him in a variety of capacities since then. Prior to rejoining EGI in 2012, Helfand was Founder and President of Helix Funds, where he oversaw the acquisition, management and disposition of more than $2.2 billion of real estate assets. While at Helix, he also served as Chief Executive Officer for American Residential Communities, a Helix portfolio company. Before founding Helix, Helfand served as Executive Vice President and Chief Investment Officer for Equity Office Properties Trust, the largest REIT in the U.S. at the time, where he led approximately $12 billion of mergers and acquisitions activity. Prior to Equity Office, Helfand served as a Managing Director and participated in the formation of Equity International. He also held the role of President and Chief Executive Officer at Equity LifeStyle Properties, and served as Chairman of the board’s audit committee. His earlier career included investment activity in a variety of asset classes, including retail, office, parking and multifamily. Helfand holds an MBA from the University of Chicago Graduate School of Business, and a BA from Northwestern University. He serves as a

member of the Board of Trustees and Executive Committee of National Louis University, as a Director of the Ann & Robert H. Lurie Children’s Hospital of Chicago, on the Executive Committee of the Zell/Lurie Real Estate Center at the Wharton School, and on the Board of Visitors at the Weinberg College of Arts and Sciences at Northwestern University.

Shareholders are urged to vote the GOLD consent card for the removal the entire Board of CommonWealth. Corvex and Related believe that the removal of ALL CommonWealth REIT Trustees is the only path to effect real change and unlock the substantial value embedded within the company’s conflicted structure. Without removal of the entire board, shareholders will continue to have a company operated by the same management team that has overseen the worst performing public suburban or CBD office REIT, as measured by virtually any metric.

Corvex and Related intend to submit to CommonWealth a record date request by February 16, after which CommonWealth will have 10 business days to set the record date. On February 10, 2014 CommonWealth announced that it has set a “conditional” record date of February 18, 2014. The consent solicitation must be concluded within 30 days of the record date.

Please note that internet voting is NOT available. Shareholders must sign, date and return the GOLD Consent Card in the pre-paid return envelopes they will be receiving. If you are a CommonWealth shareholder we urge you to call today our solicitor, D.F. King & Co., Inc., at 1-800-714-3313 to make sure we receive your consent.

Additional Information Regarding the Solicitation

Corvex Management LP and Related Fund Management, LLC have filed a definitive solicitation statement with the Securities and Exchange Commission (the “SEC”) to (1) solicit consents to remove the entire board of trustees of CommonWealth REIT (the “Removal Proposal”), and (2) elect a slate of new trustees at a special meeting of shareholders that must be promptly called in the event that the Removal Proposal is successful. Investors and security holders are urged to read the definitive solicitation statement and other relevant documents because they contain important information regarding the solicitation. The definitive solicitation statement and all other relevant documents are available, free of charge, on the SEC’s website at www.sec.gov.

The following persons are participants in connection with the solicitation of CommonWealth REIT shareholders: Corvex Management LP, Keith Meister, Related Fund Management, LLC, Related Real Estate Recovery Fund GP-A, LLC, Related Real Estate Recovery Fund GP, L.P., Related Real Estate Recovery Fund, L.P., RRERF Acquisition, LLC, Jeff T. Blau, Richard O’Toole, David R. Johnson, James Corl, Edward Glickman, Peter Linneman, Jim Lozier, Kenneth Shea, EGI-CW Holdings, L.L.C., David Helfand and Samuel Zell. Information regarding the participants in the solicitation and a description of their direct and indirect interests, by security holdings or otherwise, to the extent applicable, is available in the definitive solicitation statement filed with the SEC on January 28, 2014 and Supplement No. 1 to be filed with the SEC.

About Corvex Management LP

Corvex Management LP is an investment firm headquartered in New York, New York that engages in value-based investing across the capital structure in situations with identifiable catalysts. Corvex was founded in March 2011 and follows an opportunistic approach to investing with a specific focus on equity investments, special situations and distressed securities largely in North America.

About Related Fund Management LLC

Related Fund Management, LLC is an affiliate of Related Companies, one of the most prominent privately-owned real estate firms in the United States. Formed 40 years ago, Related is a fully integrated, highly diversified industry leader with experience in virtually every aspect of development, acquisitions, management, finance, marketing and sales. Related’s existing portfolio of real estate assets, valued at over $15 billion, is made up of best-in-class mixed-use, residential, retail, office and affordable properties. For more information about Related Companies please visit www.related.com.

For further information, contact:

Rupal Doshi

Corvex

(212) 474-6750

rdoshi@corvexcap.com

Joanna Rose

Related

(212) 801-3902

jrose@related.com

Terry Holt

Equity Group Investments

Office: (312) 466-3979

Mobile: (312) 925-5387

tholt@egii.com

INVESTORS:

Edward McCarthy / Richard Grubaugh

D.F. King & Co., Inc.

(212) 269-5550

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