<PAGE>

                                                                 EXHIBIT 10.51.2

                       FIRST AMENDMENT TO LEASE AGREEMENT

      THIS FIRST AMENDMENT TO LEASE AGREEMENT (this "First Amendment") is
entered into as of December 4, 2003 by and between SNH ALT LEASED PROPERTIES
TRUST, a Maryland real estate investment trust ("Landlord"), and AHC TRAILSIDE,
INC., a Delaware corporation ("Tenant").

                                   WITNESSETH:

      WHEREAS, Landlord and Tenant are parties to that certain Lease Agreement
dated as of February 28, 2003 (the "Lease"), pursuant to which Landlord leased
the Leased Property (this and other capitalized terms used herein and not
otherwise defined having the meanings ascribed to such terms in the Lease) to
Tenant and Tenant leased the Leased Property from Landlord, all as more
particularly described in the Lease; and

      WHEREAS, SNH ALT Mortgaged Properties Trust ("Lender") and Pomacy
Corporation ("Borrower") are parties to that certain Loan Agreement, dated as of
February 28, 2003 (the "Loan Agreement"), pursuant to which Lender agreed to
loan up to the principal amount of SIX MILLION NINE HUNDRED THOUSAND DOLLARS
($6,900,000) (the "Loan") to Borrower and Borrower agreed to borrow up to such
principal amount from Lender, all as more particularly described in the Loan
Agreement; and

      WHEREAS, the Lease Documents and the Loan Documents (as defined in the
Loan Agreement) are cross-defaulted and cross-collateralized; and

      WHEREAS, Landlord and Lender are both wholly-owned subsidiaries of Senior
Housing Properties Trust ("SNH"); and

      WHEREAS, Tenant and Borrower are both wholly-owned subsidiaries of Alterra
Healthcare Corporation ("Alterra"); and

      WHEREAS, Landlord, Lender, Tenant, Borrower, Alterra and FIT-ALT SNH Loan
LLC ("FIT") have entered into that certain Tri-Party Agreement of even date
herewith (the "Tri-Party Agreement") pursuant to which Lender has agreed to sell
the Loan to FIT, and FIT has agreed to purchase the Loan from Lender, all as
more particularly described in the Tri-Party Agreement; and

      WHEREAS, in connection with the transactions contemplated by the Tri-Party
Agreement, Landlord and Tenant have agreed to amend the Lease Documents to
remove all provisions related to the Borrower, the Loan Agreement and the Loan
Documents,

<PAGE>

including, without limitation, the provisions which cross-default and
cross-collateralize the Lease Documents with the Loan Documents;

      NOW, THEREFORE, in consideration of the foregoing and for other
consideration, the mutual receipt and legal sufficiency of which are hereby
acknowledged, Landlord and Tenant hereby agree as follows:

      1. The following definitions are hereby amended by deleting such
definitions in their entirety and inserting "[INTENTIONALLY DELETED]"
immediately after the section reference in lieu thereof:

       Section 1.13  "Borrower"
       Section 1.14  "Borrower Indebtedness"
       Section 1.70  "Lender"
       Section 1.72  "Loan Agreement"
       Section 1.73  "Loan Documents"

      2. The definition of "Acquiring Guarantor" set forth in Section 1.2 of the
Lease is hereby amended by deleting the existing definition in its entirety and
inserting the following in its place:

      "Acquiring Guarantor" shall mean any successor in interest to any
      Guarantor, which shall be deemed to include, without limitation, (a) any
      Person or group of two or more Persons acting in concert (other than a
      Permitted Control Person or a group comprised exclusively of Permitted
      Control Persons), that acquire the ownership of fifty percent (50%) or
      more of the voting power of the outstanding shares of voting stock of any
      Guarantor, (b) any Person resulting from the merger or consolidation of
      any Guarantor, and (c) any Person acquiring through any one or more sales
      or conveyances all or substantially all of any Guarantor's assets
      (including its capital stock) or business.

      3. The definition of "Change in Control" set forth in Section 1.18 of the
Lease is hereby amended by deleting the existing definition in its entirety and
inserting the following in its place:

      "Change in Control" shall mean (a) the acquisition by any Person, or two
      or more Persons acting in concert, of record ownership of, or the right to
      vote, or the

                                     - 2 -
<PAGE>

            power to direct the vote of, in excess of fifty percent (50%) of the
            voting power of the outstanding shares of voting stock of Tenant or
            any Guarantor, as the case may be, other than pursuant to a
            Permitted Merger or a Secured Merger, (b) the merger or
            consolidation of Tenant or any Guarantor with or into any other
            Person (other than a Permitted Merger, a Secured Merger or a merger
            or consolidation of any Person with or into Tenant or any Guarantor
            that does not result in a Change in Control of Tenant or such
            Guarantor under clauses (a), (c) or (d) of this definition), (c) any
            one or more sales or conveyances to any Person of all or
            substantially all of its assets (including capital stock) or
            business of Tenant or any Guarantor, as the case may be, or (d) the
            cessation, for any reason, of the individuals who, on the
            Commencement Date, constituted the board of directors of Tenant or
            any Guarantor (together with any new directors elected or appointed
            pursuant to, and on the effective date of, a Conforming Plan and any
            new directors whose election by such board or whose nomination for
            election by the shareholders of Tenant or such Guarantor, as the
            case may be, was approved by a vote of a majority of the directors
            then still in office who were either directors on the Commencement
            Date or the effective date of a Conforming Plan as aforesaid or
            whose election or nomination for election was previously so
            approved) to constitute a majority of the board of directors of
            Tenant or such Guarantor then in office; provided, however, that no
            Change in Control shall be deemed to have occurred as a result or
            arising out of: (i) any change in ownership resulting from the
            issuance of equity securities in connection with the confirmation of
            any Conforming Plan or (ii) any acquisition referred to in clause
            (a) of this definition with respect to the voting power of the
            outstanding shares of voting stock of any Parent of Tenant so long
            as such Parent is a Guarantor and at the time of such acquisition
            and immediately thereafter such Parent has a consolidated Tangible
            Net Worth at least equal to Fifty Million Dollars ($50,000,000).

      4. The definition of "Guarantor" set forth in Section 1.51 of the Lease is
hereby amended by deleting the existing definition in its entirety and inserting
the following in its place:

                                     - 3 -
<PAGE>

            "Guarantor" shall mean Alterra and each and every other guarantor of
            Tenant's obligations under this Agreement, and each such guarantor's
            successors and assigns, and shall expressly exclude Pomacy
            Corporation to the extent that the Guaranty Agreement (Pomacy
            Corporation), dated as of February 28, 2003, given by Pomacy
            Corporation to Landlord, has been terminated.

      5. The definition of "Guaranty" set forth in Section 1.52 of the Lease is
hereby amended by deleting the existing definition in its entirety and inserting
the following in its place:

            "Guaranty" shall mean any guaranty agreement executed by a Guarantor
            in favor of Landlord pursuant to which the payment or performance of
            Tenant's obligations under this Agreement are guaranteed, together
            with all modifications, amendments and supplements thereto, but
            expressly excluding the Guaranty Agreement (Pomacy Corporation),
            dated as of February 28, 2003, given by Pomacy Corporation to
            Landlord, to the extent the same has been terminated.

      6. The definition of "Conforming Plan" set forth in Section 1.25 of the
Lease is hereby amended by deleting the phrase "the Loan Agreement and the other
Loan Documents" from subsection (a) thereof.

      7. Section 12.1 ("Events of Default") is hereby amended by:

            (a) deleting subsection (f) in its entirety and inserting
"[INTENTIONALLY DELETED]" in its place.

            (b) deleting subsection (g) in its entirety and inserting the
following in its place:

            should Tenant or any Guarantor generally not be paying its debts as
            they become due or should Tenant or any Guarantor make a general
            assignment for the benefit of creditors; provided, however that the
            failure of Alterra to generally not be paying its debts as they
            become due or the making of a general assignment by Alterra for the
            benefit of creditors shall not constitute an Event of Default
            hereunder if such failure or assignment occurs on or prior to
            January 31, 2003 or at any time during the pendency of

                                     - 4 -
<PAGE>


            a Conforming Bankruptcy Proceeding filed on or prior to January 31,
            2003; or

            (c) deleting subsection (i) in its entirety and inserting the
following in its place:

            should Tenant or any Guarantor cause or institute any proceeding for
            its dissolution or termination; or

      8. Section 21.1 ("Prompt Payment of Indebtedness") is hereby amended by
deleting it in its entirety and inserting the following in its place:

            Prompt Payment of Indebtedness. Tenant shall (a) pay or cause to be
            paid when due all payments of principal of and premium and interest
            on Tenant's Indebtedness for money borrowed and shall not permit or
            suffer any such Indebtedness to become or remain in default beyond
            any applicable grace or cure period, (b) pay or cause to be paid
            when due all lawful claims for labor and rents with respect to the
            Leased Property, (c) pay or cause to be paid when due all trade
            payables and (d) pay or cause to be paid when due all other of
            Tenant's Indebtedness upon which it is or becomes obligated, except,
            in each case, other than that referred to in clause (a), to the
            extent payment is being contested in good faith by appropriate
            proceedings in accordance with Article 8 and if Tenant shall have
            set aside on its books adequate reserves with respect thereto in
            accordance with GAAP, if appropriate, or unless and until
            foreclosure, distraint sale or other similar proceedings shall have
            been commenced.

      9. Section 21.5 ("Indebtedness") is hereby amended by deleting subsection
(a) in its entirety and inserting "[INTENTIONALLY DELETED]" in its place.

      10. Section 21.10 ("Liens and Encumbrances") is hereby amended by deleting
subsection (d) in its entirety and inserting the following in its place:

            To secure Indebtedness permitted pursuant to Section(s) 21.5
            (b)-(e).

      11. Landlord hereby represents and warrants that there are no Facility
Mortgagees or other lenders or ground lessors arising by, through or under
Landlord holding any mortgage or

                                     - 5 -
<PAGE>

other lien, encumbrance or interest in the Lease, the Rent payable thereunder or
in the Leased Property.

      12. Except as otherwise expressly stated (i) herein or (ii) in any
document or instrument executed and delivered in connection herewith or (iii) in
the Terms of Settlement among Alterra, FEBC-ALT Investors LLC, FEBC-ALT
Investors Inc., FEBC-ALT Holdings Inc., FEBC-ALT Acquisition Inc., Landlord and
Lender (the "Settlement") attached as Exhibit A to the Findings of Fact,
Conclusions of Law and Order under 11 U.S.C. Section 1129(a) and (b) and Fed. R.
Bankr. P. 3020 Confirming Second Amended Plan of Reorganization for Alterra
Healthcare Corporation under Chapter 11 of the Bankruptcy Code (the
"Confirmation Order"), which Confirmation Order, including Exhibit A, was
approved by the United States Bankruptcy Court for the District of Delaware on
November 26, 2003 ((i) - (iii) are hereinafter collectively referred to as the
"December 2003 Documents"), nothing in this First Amendment shall affect,
modify, release, limit or otherwise impair any rights or obligations of any of
the parties hereto under the Lease or the Lease Documents.

      13. This First Amendment shall be effective as of the Effective Date (as
such term is defined in the Tri-Party Agreement) and no sooner.

                            [SIGNATURE PAGES FOLLOW]

                                     - 6 -
<PAGE>

      IN WITNESS WHEREOF, the parties hereto have executed this First Amendment
under seal as of the date first written above.

                                         LANDLORD:

                                         SNH ALT LEASED PROPERTIES TRUST,
                                         a Maryland real estate investment trust

                                         By: /s/ John R. Hoadley
                                             -----------------------------------
                                         Name: John R. Hoadley
                                               ---------------------------------
                                         Title: Treasurer
                                                --------------------------------

                                         TENANT:

                                         AHC TRAILSIDE, INC.,
                                         a Delaware corporation

                                         By: /s/ Mark W. Ohlendorf
                                             -----------------------------------
                                         Name: Mark W. Ohlendorf
                                               ---------------------------------
                                         Title: Vice President
                                                --------------------------------

                                     - 7 -