Document:

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                                                                    Exhibit 10.7

                  FIRST AMENDMENT TO STOCK PURCHASE AGREEMENT

          THIS FIRST AMENDMENT TO STOCK PURCHASE AGREEMENT (this "Amendment") is
entered into by and between CarrAmerica Realty Corporation ("Seller") and
FrontLine Capital Group (formerly known as Reckson Services Industries, Inc.)
("Buyer") as of April 29, 2000.

                                   RECITALS:

          WHEREAS, Seller and Buyer entered into a Stock Purchase Agreement
dated as of January 20, 2000 (such agreement as heretofore, hereby and
hereinafter amended, the "Stock Purchase Agreement") pursuant to which Seller
agreed to sell shares of common stock of HQ Global Workplaces, Inc. (the
"Company") owned by it to Buyer on the terms set forth therein;

          WHEREAS, the parties hereto wish to make certain amendments to the
Stock Purchase Agreement; and

          WHEREAS, capitalized terms not otherwise defined herein shall have the
meanings set forth in the Stock Purchase Agreement.

          NOW, THEREFORE, in consideration of the foregoing and the respective
covenants and agreements set forth herein, the parties, intending to be legally
bound, agree as follows:

          1.   The second sentence of Section 4(c)(ii) of the Stock Purchase
Agreement is hereby deleted in its entirety and replaced by the following:

          "Other than (i) this Agreement, (ii) the Amended and Restated
          Stockholders Agreement dated as of September 29, 1998 by and among the
          Company, Seller and certain other stockholders of the Company (the
          "Existing Stockholders Agreement"), and (iii) any stockholders
          agreement to be executed in connection with the Closing hereunder,
          none of the Shares are subject to any agreement, contract, commitment,
          understanding or arrangement, including any such agreement, contract,
          commitment, understanding or arrangement restricting or otherwise
          relating to the voting, dividend rights or disposition of the Shares."

          2.   The last sentence of Section 5(c) of the Stock Purchase Agreement
is hereby deleted in its entirety and replaced with the following:
<PAGE>

          "No consent, approval, license, permit, order or authorization of, or
          registration, declaration or filing with, any Governmental Entity is
          required to be obtained or made by or with respect to the Buyer in
          connection with the execution, delivery, performance of this Agreement
          by the Buyer or the consummation of the transactions contemplated
          hereby."

          3.   Section 5 of the Stock Purchase Agreement is hereby amended by
adding the following section:

          "(d) Investment Representation.  Each party acquiring Shares
               -------------------------
          hereunder will be receiving the Shares for his or its own account for
          investment only and not with a view towards distribution or resale.
          Each party acquiring Shares hereunder will either be an "accredited"
          investor within the meaning of Rule 501 promulgated under the
          Securities Act of 1933, as amended (the "Securities Act"), or will
          have such knowledge and experience in financial and business matters
          that such party is capable of evaluating the merits and risks of
          investment in the Shares and will be able to bear the economic risk of
          its investment in the Shares.  Each party acquiring Shares hereunder
          acknowledges that any routine sale of such Shares made in reliance
          upon Rule 144 promulgated under the Securities Act can be made only in
          accordance with the terms and conditions of such Rule and further,
          that in case such Rule is not applicable to any sale of the Shares,
          resale thereof may require compliance with some other exemption under
          the Securities Act prior to resale.  Each party acquiring Shares
          hereunder acknowledges that the certificates representing the Shares
          issued pursuant to this Agreement shall bear the following legend:

          "THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
          1933. SUCH SECURITIES MAY NOT BE SOLD OR OFFERED FOR SALE,
          TRANSFERRED, HYPOTHECATED OR OTHERWISE ASSIGNED IN THE ABSENCE OF AN
          EFFECTIVE REGISTRATION STATEMENT WITH RESPECT THERETO UNDER SUCH ACT
          OR AN OPINION OF COUNSEL THAT AN EXEMPTION FROM REGISTRATION FOR SUCH
          SALE, OFFER, TRANSFER, HYPOTHECATION OR OTHER ASSIGNMENT IS AVAILABLE
          UNDER SUCH ACT.""

          4.   Section 12(a)(iv) is hereby deleted in its entirety and replaced
with the following:

                                       2
<PAGE>

          "by the Seller or Buyer (provided, however, in the case of Buyer, only
          if Buyer has given the Seller at least five business days' prior
          written notice of its intention to terminate pursuant to this Section
          12(a)(iv), which notice may not be given prior to June 1, 2000), if
          the Closing does not occur on or prior to May 31, 2000;"

          5.   Section 24 is hereby deleted in its entirety and replaced with
the following:

          "24. Additional Signature Parties.  At any time after the date
               ----------------------------
          hereof and at or prior to five (5) business days prior to the Closing,
          any other holder of Non-Voting Common Stock, any holder of voting
          common stock, par value $.01 per share of the Company (the "Voting
          Common Stock") and any holder of warrants to acquire shares of Non-
          Voting Common Stock (the "Warrants") as of the date hereof shall be
          permitted to execute and deliver to the Buyer an Additional Party
          Signature Page substantially in the form of Annex B hereto (each such
          party who executes and delivers to the Buyer such document, an
          "Additional Stockholder Party"), pursuant to which such Additional
          Stockholder Party shall be entitled, subject to the terms and
          conditions herein, to become a "Seller" for all purposes of this
          Agreement, and shall be entitled to sell to Buyer the number of shares
          of Non-Voting Common Stock or Voting Common Stock or the number of
          Warrants, held by such Additional Stockholder Party and set forth on
          such Additional Party Signature Page on the same terms set forth in
          this Agreement; provided that each holder of Warrants that elects to
          sell Warrants hereunder shall be entitled to receive a price per
          Warrant, equal to the Per Share Purchase Price, as adjusted hereunder,
          minus the exercise price of such Warrant.  Each Additional Stockholder
          Party shall be deemed to have made the representations and warranties
          of Seller herein and be bound by the other agreements of Seller
          herein, and the shares of Non-Voting Common Stock and Voting Common
          Stock and the Warrants sold by each such party shall be deemed
          "Shares" for all purposes hereunder.  To the extent that any
          Additional Stockholder Party elects to sell shares of Non-Voting
          Common Stock, shares of Voting Common Stock or Warrants, the number of
          shares of Non-Voting Common Stock to be sold by CarrAmerica pursuant
          to this Agreement shall be decreased by the number of shares of Non-
          Voting Common Stock, shares of Voting Common Stock or Warrants, as
          applicable, to be sold by such Additional Stockholder Party, it being
          intended that the Buyer shall be entitled to and required to purchase
          only the number of shares of Non-Voting Common Stock, shares of Voting
          Common Stock and Warrants, in the aggregate from

                                       3
<PAGE>

          CarrAmerica and all Additional Stockholder Parties, shown as being
          proposed to be sold by CarrAmerica on Exhibit A as delivered on
          January 20, 2000; provided that, notwithstanding anything to the
          contrary contained herein, in the event any Additional Stockholder
          Party shall breach any representation or warranty or otherwise fail to
          perform any of its obligations under this Agreement, CarrAmerica shall
          have the right to make such representation or warranty and otherwise
          perform such obligations on behalf of such Additional Stockholder
          Party. If Warrants are sold to Buyer pursuant to this Section 24,
          Buyer will immediately exercise such Warrants and pay the exercise
          price therefor to the Company (or its successors)."

          6.   Simultaneously with the execution of this Amendment, RSI shall
contribute $1,350,000 to VANTAS and VANTAS shall pay $1,350,000 in cash (the
"Company Stockholder Extension Payment") to the Company, as agent for the
stockholders of the Company who have elected to sell all or a portion of their
shares in the Company, as compensation for the extension of the Closing Date
from April 30, 2000 to May 31, 2000, which funds shall be distributed by the
Company on a pro rata basis to such stockholders based on the percentage of
shares being sold by each such stockholder divided by the total number of shares
being sold. The Company Stockholder Extension Payment is nonrefundable and will
not be used to adjust the consideration otherwise payable in connection with the
consummation of the transactions contemplated by the Stock Purchase Agreement.

          7.   Seller represents and warrants that Seller has the requisite
capacity and authority, and has taken all actions necessary in order, to
execute, deliver and perform its obligations under this Amendment.  This
Amendment is a legal, valid and binding obligation of Seller, enforceable in
accordance with its terms, except insofar as enforcement thereof may be limited
by bankruptcy, insolvency or other laws relating to or affecting enforcement of
creditors' rights generally including such general equitable principles as may
apply in the enforcement of creditors' rights.

          8.   Buyer represents and warrants that Buyer has the requisite
capacity and authority, and has taken all actions necessary in order, to
execute, deliver and perform its obligations under this Amendment.  This
Amendment is a legal, valid and binding obligation of Buyer, enforceable in
accordance with its terms, except insofar as enforcement thereof may be limited
by bankruptcy, insolvency or other laws relating to or affecting enforcement of
creditors' rights generally including such general equitable principles as may
apply in the enforcement of creditors' rights.

                                       4
<PAGE>

          9.   This Amendment and the other agreements referred to herein
represent the entire understanding of the parties with respect to the subject
matter contained herein.  This Amendment may not be amended, modified or waived
except in a writing signed by the party against whom enforcement of such
amendment, modification or waiver is sought.  This Amendment shall be construed
and interpreted in accordance with the laws of the State of Delaware, without
reference to the conflict of laws principles contained therein.  This Amendment
may be executed in one or more counterparts, each of which shall be deemed an
original, but all of which, when taken together, shall constitute one and the
same instrument.

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<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be duly executed as of the date first written above.

                            CARRAMERICA REALTY CORPORATION

                            By:   /s/ Karen B. Dorigan
                               -----------------------
                            Name: Karen B. Dorigan
                            Title: Managing Director

                            FRONTLINE CAPITAL GROUP

                            By:  /s/ Jason Barnett
                               -------------------
                            Name: Jason Barnett
                            Title: Executive Vice President

                                       6<PAGE>

                                                                    Exhibit 10.8

                  FIRST AMENDMENT TO STOCK PURCHASE AGREEMENT

          THIS FIRST AMENDMENT TO STOCK PURCHASE AGREEMENT (this "Amendment") is
entered into by and among CarrAmerica Realty Corporation ("CarrAmerica"),
OmniOffices (UK) Limited ("Omni UK"), OmniOffices (Lux) 1929 Holding Company
S.A. ("LuxCo"), VANTAS Incorporated ("VANTAS")  and FrontLine Capital Group
(formerly known as Reckson Services Industries, Inc.) ("RSI") as of April 29,
2000.

                                   RECITALS:

          WHEREAS, CarrAmerica, Omni UK, LuxCo, VANTAS and RSI entered into a
Stock Purchase Agreement dated as of January 20, 2000 (such agreement, as
heretofore, hereby and hereinafter amended, the "UK Agreement") pursuant to
which CarrAmerica agreed to sell shares of common stock of Omni UK and LuxCo
owned by it together with its interest in certain loans made by it to Omni UK
and LuxCo to VANTAS on the terms set forth therein;

          WHEREAS, VANTAS, RSI, CarrAmerica, Omni UK and LuxCo have executed an
Agreement dated as of March 13, 2000, as amended, pursuant to which such parties
agreed to authorize certain actions governed by the UK Agreement;

          WHEREAS, the parties hereto wish to make certain additional amendments
to the UK Agreement; and

          WHEREAS, capitalized terms not otherwise defined herein shall have the
meanings set forth in the UK Agreement.

          NOW, THEREFORE, in consideration of the foregoing and the respective
covenants and agreements set forth herein, the parties, intending to be legally
bound, agree as follows:

          1.   The second and third recitals of the UK Agreement are hereby
deleted in their entirety and replaced with the following:

          "WHEREAS, CarrAmerica has made certain loans to Omni UK and LuxCo as
          described on Exhibit B attached hereto, and may make additional loans
          to Omni UK and LuxCo as contemplated by Section 4.1(b)(iii) hereof
          (collectively, the "Loans"), which Loans are or will be evidenced by
          promissory notes, the date and denominations of which as of the date
          hereof are set forth on Exhibit B (the "Notes");
<PAGE>

          WHEREAS, CarrAmerica desires to sell and transfer to VANTAS and VANTAS
          desires to purchase from CarrAmerica (i) the Notes in exchange for an
          amount equal to the principal amount of, and any accrued and unpaid
          interest with respect to, the Notes outstanding as of the Closing Date
          in accordance with Section 1.2 (the "Loan Consideration"), and (ii)
          all of the shares of non-voting common stock, par value (Pounds).01
          per share, of Omni UK (which represents approximately 95% of the
          equity interest in Omni UK) (the "UK Non-Voting Common Stock") and all
          of the shares of non-voting common stock, par value $250 per share, of
          LuxCo (which represents approximately 95% of the equity interest in
          LuxCo) (the "Lux Non-Voting Common Stock"; and together with the UK
          Non-Voting Common Stock, collectively, the "Shares") owned of record
          by the Seller for total consideration equal to the "Share
          Consideration" as calculated pursuant to Annex A (the "Share
          Consideration");"

          2.   Section 2.3 of the UK Agreement is hereby amended by inserting
the following at the end of Section 2.3:

          "Notwithstanding anything to the contrary contained herein, the
          parties acknowledge that, in connection with the consummation of the
          transactions contemplated by the Merger Agreement, the consent of the
          general partner of Mercury Executive Offices, L.P. may be required
          under the Limited Partnership Agreement dated November 16, 1998 and
          the management agreements related thereto; it being understood that
          Omni UK, LuxCo, VANTAS and RSI shall use commercially reasonable
          efforts to obtain such consent."

          3.   Section 2.12(a) of the UK Agreement is hereby amended by deleting
the first sentence in its entirety and replacing it with the following:

          "Neither of the Companies nor any of their respective Subsidiaries
          owns any real property or interests in real property, other than
          OmniUK Real Property Leases (as defined below), LuxCo Real Property
          Leases (as defined below), the OmniUK Owned Property (as defined
          below) and certain interests in certain real property assets held
          directly or indirectly through the MAM Investment (as defined below)."

          4.   The provision immediately following Section 2.13(j) on page 12 of
the UK Agreement is hereby deleted in its entirety and replaced with the
following:

                                       2
<PAGE>

          "; provided, however, that notwithstanding anything to the contrary
          contained herein other than any contracts relating to office
          equipment, maintenance or service for any individual business center
          of the Companies or their respective Subsidiaries which does not
          require annual payments in excess of $25,000, the sum of the aggregate
          future liabilities and costs of termination associated with all
          contracts, arrangements, agreements or understandings to which either
          of the Companies or any of their respective Subsidiaries is a party or
          by which they or any of their respective properties or assets which
          are not set forth on Schedule 5.2, 2.13 or 2.16 is bound does not
          exceed $5,000,000."

          5.   Section 2.28 of the UK Agreement is hereby deleted in its
entirety and replaced with the following:

          "Section 2.28 MAM Investment.  OmniUK and certain of its Subsidiaries
                        --------------
          have agreed to invest up to (Pounds)20 million in Mercury Executive
          Offices, L.P. on the terms and conditions set forth in the limited
          partnership agreement, dated November 16, 1998 (the "MAM Investment"),
          of which at least (Pounds)15,594,241 has been funded. Notwithstanding
          anything to the contrary contained herein, the parties acknowledge
          that, in connection with the consummation of the transactions
          contemplated by the Merger Agreement, the consent of the general
          partner of Mercury Executive Offices, L.P. may be required under the
          Limited Partnership Agreement dated November 16, 1998 and the
          management agreements related thereto; it being understood that Omni
          UK, LuxCo, VANTAS and RSI shall use commercially reasonable efforts to
          obtain such consent."

          6.   Section 4.1(b)(iii) of the UK Agreement is hereby deleted in its
entirety and replaced with the following:

          "incur or assume any liabilities or incur any indebtedness for
          borrowed money, assume, guarantee, endorse or otherwise as an
          accommodation become responsible for the obligations of any other
          individual, corporation or entity (other than a wholly owned
          Subsidiary) except pursuant to contracts or agreements listed in
          Schedule 2.13 or except for loans and equity investments (which shall
          be deemed to be included on Exhibit A or Exhibit B, as applicable,
          attached hereto) by CarrAmerica necessary to fund the business of the
          Companies in the ordinary course or to fund committed developments;"

          7.   Section 4.1(b)(v) of the UK Agreement is hereby deleted in its
entirety and replaced with the following:

                                       3
<PAGE>

          "make any material change in any Omni UK Real Property Lease or LuxCo
          Real Property Lease, as applicable, or other contracts or enter into,
          renew, fail to renew, terminate or permit to be terminated any Omni UK
          Real Property Lease or LuxCo Real Property Lease, as applicable, any
          other contract or agreement that calls for aggregate annual payments
          of $25,000 or more and which either (i) is not terminable by either of
          the Companies or their respective Subsidiary, as applicable, at will
          on 60 days or less notice without payment of a penalty or (ii) has a
          term of more than six months, other than the operating leases and
          related letters of comfort executed by Omni UK, LuxCo or any
          Subsidiaries thereof with respect to development and refurbishment
          projects identified in the CapEx Budget, as contemplated by the
          operating budgets for those projects;"

          8.   Section 5.5 of the UK Agreement is hereby amended by inserting
the following sentence at the end of such section:

          "Not later than six months after the Closing, the Second Step
          Surviving Corporation shall reimburse CarrAmerica, as agent for itself
          and the stockholders of HQ Global Workplaces, Inc. ("HQ Global")
          immediately prior to the Closing (based on their proportionate
          indirect beneficial interests in Omni UK and LuxCo), for any
          additional lease deposits of Omni UK and LuxCo that are not identified
          on the disclosure schedules delivered in connection with the execution
          of this Agreement; provided that the Second Step Surviving Corporation
          is provided with a schedule of such additional lease deposits at or
          prior to Closing."

          9.   Article 5 of the UK Agreement is hereby amended by adding the
following sections at the end of such Article 5:

          "Section 5.6  MAM Hammersmith Center.  RSI and VANTAS hereby consent
                        ----------------------
          to the development of the MAM Hammersmith center. Immediately
          subsequent to the Closing, the Second Step Surviving Corporation shall
          reimburse CarrAmerica, as agent for itself and the stockholders of HQ
          Global immediately prior to the Closing (based on their proportionate
          indirect beneficial interests in Omni UK and LuxCo), for any costs
          that have been or will be incurred prior to Closing in connection with
          the development of the MAM Hammersmith center; provided that the
          Second Step Surviving Corporation is provided with a schedule of such
          amounts at or prior to Closing.

                                       4
<PAGE>

          Section 5.7  Prepaid Rent; Tenant Reimbursement. Immediately
                       ----------------------------------
          subsequent to the Closing, the Second Step Surviving Corporation shall
          pay to CarrAmerica, as agent for itself and the stockholders of HQ
          Global immediately prior to the Closing (based on their proportionate
          indirect beneficial interests in Omni UK and LuxCo), $576,000, which
          represents the amount of rent of Omni UK and LuxCo that was prepaid
          for the period after Closing;; provided that the Second Step Surviving
          Corporation is provided with a schedule of such amounts at or prior to
          Closing.

          Section 5.8  Capital Expenditures and Cash Fundings. Immediately
          ---------------------------------------------------
          subsequent to the Closing, the Second Step Surviving Corporation shall
          reimburse CarrAmerica, as agent for itself and the stockholders of HQ
          Global immediately prior to the Closing (based on their proportionate
          indirect beneficial interests in Omni UK and LuxCo), for all capital
          expenditures and development expenses of Omni UK or LuxCo incurred
          after April 30, 2000 and prior to Closing, including amounts funded to
          enable Omni UK to satisfy its capital commitments with respect to the
          MAM Investment; provided that the Second Step Surviving Corporation is
          provided with a schedule of such amounts at or prior to Closing.

          Section 5.9  Side Letter.  At Closing, VANTAS shall assume the
                       -----------
          obligations of CarrAmerica with respect to that certain Side Letter
          dated as of November 16, 1998 by and among HQMerc UK Management
          Limited ("HQ Merc"), P.R.A. Investments Limited ("PRA"), P.K.
          Investments Limited ("PKL"), Peter Allport ("Allport"), Peter Kershaw
          ("Kershaw"), HQ Holdings Limited ("HQ Holdings") and CarrAmerica and
          that certain Side Letter dated as of November 16, 1998 by and among HQ
          Merc UK Partnership Limited, PRA, PKL, Allport, Kershaw, HQ Holdings
          and CarrAmerica."

          10.  Section 6.2(b) of the UK Agreement is hereby deleted in its
entirety and replaced with the following:

          "CarrAmerica shall have received the cash required by Sections 1.2,
          5.3, 5.5, 5.6, 5.7 and 5.8."

          11.  All references to "April 30, 2000" in Article X(a)(iv), Article
X(a)(vi) and Article X(a)(vii) of the UK Agreement are hereby changed to "May
31, 2000."

          12.  All references to "May 1, 2000" in Article X(a)(iv) of the UK
Agreement are hereby changed to "June 1, 2000."

                                       5
<PAGE>

          13.  The amount of the Loan Notes identified in Annex A to the UK
Agreement is hereby deleted and replaced with "21,229,392 (reduced by the amount
of any payments thereto prior to Closing)."

          14.  Simultaneously with the execution of this Amendment, RSI shall
contribute $900,000 to VANTAS and VANTAS shall pay $900,000 in cash (the "UK
Stockholder Extension Payment") to CarrAmerica, as compensation for the
extension of the Closing Date from April 30, 2000 to May 31, 2000. The UK
Stockholder Extension Payment is nonrefundable and will not be used to adjust
the consideration otherwise payable in connection with the consummation of the
transactions contemplated by the UK Agreement.

          15.  CarrAmerica, Omni UK and LuxCo represent and warrant, each as to
itself only, that each of CarrAmerica, Omni, UK and LuxCo has the requisite
capacity and authority, and has taken all actions necessary in order, to
execute, deliver and perform its obligations under this Amendment.  This
Amendment is a legal, valid and binding obligation of each of CarrAmerica, Omni
UK and LuxCo, enforceable in accordance with its terms, except insofar as
enforcement thereof may be limited by bankruptcy, insolvency or other laws
relating to or affecting enforcement of creditors' rights generally including
such general equitable principles as may apply in the enforcement of creditors'
rights.

          16.  VANTAS and RSI represent and warrant that each of VANTAS and RSI
has the requisite capacity and authority, and has taken all actions necessary in
order, to execute, deliver and perform its obligations under this Amendment.
This Amendment is a legal, valid and binding obligation of each of VANTAS and
RSI, enforceable in accordance with its terms, except insofar as enforcement
thereof may be limited by bankruptcy, insolvency or other laws relating to or
affecting enforcement of creditors' rights generally including such general
equitable principles as may apply in the enforcement of creditors' rights.

          17.  This Amendment, the Merger Agreement, as amended, the Stock
Purchase Agreement, as amended, and the other agreements referred to herein and
therein represent the entire understanding of the parties with respect to the
subject matter contained herein.  This Amendment may not be amended, modified or
waived except in a writing signed by the party against whom enforcement of such
amendment, modification or waiver is sought.  This Amendment shall be construed
and interpreted in accordance with the laws of the State of Delaware, without
reference to the conflict of laws principles contained therein.  This Amendment
may be executed in one or more counterparts, each of which shall be deemed an
original, but all of which, when taken together, shall constitute one and the
same instrument.

                                       6
<PAGE>

                           [signature page follows]

                                       7
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be duly executed as of the date first written above.

                         CARRAMERICA REALTY CORPORATION

                         By:   /s/ Karen B. Dorigan
                            -----------------------
                         Name: Karen B. Dorigan
                         Title: Managing Director

                         OMNIOFFICES (UK) LIMITED

                         By:   /s/ Thomas A. Carr
                            ---------------------
                         Name: Thomas A. Carr
                         Title: Director

                         OMNIOFFICES (LUX) 1929 HOLDING COMPANY S.A.

                         By:   /s/ Thomas A. Carr
                            ---------------------
                         Name: Thomas A. Carr
                         Title: Director

                         FRONTLINE CAPITAL GROUP

                         By:   /s/ Jason Barnett
                            --------------------
                         Name: Jason Barnett
                         Title: Executive Vice President

                         VANTAS INCORPORATED

                         By:   /s/ Steven M. Rathkopf
                            -------------------------
                         Name: Steven M. Rathkopf
                         Title: Secretary

                                       8

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