<PAGE>

                                                                   Exhibit 10.21

District of Columbia
Loan No. C-332339

                      CONSOLIDATED, AMENDED AND RESTATED
                                PROMISSORY NOTE
                                ---------------

$183,700,000.00                                             As of March 19, 1999

          THIS CONSOLIDATED, AMENDED AND RESTATED PROMISSORY NOTE (this "Note")
is made by CARR REALTY, L.P., a Delaware limited partnership, hereinafter called
"Borrower", for the benefit of THE NORTHWESTERN MUTUAL LIFE INSURANCE COMPANY, a
Wisconsin corporation, 720 East Wisconsin Avenue, Milwaukee, Wisconsin 53202,
who, together with any subsequent holder of this Note, is hereinafter referred
to as "Lender", in substitution for and in replacement of, but not in repayment
of, the following notes, all of which Lender holds (which notes are hereinafter
referred to as the "Existing Notes"):

          1.   Promissory Note dated February 1, 1993 in the original principal
               amount of $80,000,000.00 made by Borrower and payable to the
               order of Lender, as amended by that certain First Amendment to
               Promissory Note dated October 12, 1995 between Borrower and
               Lender and by that certain Second Amendment to Promissory Note
               dated April 26, 1996 between Borrower and Lender.

          2.   Promissory Note dated September 16, 1993 in the original
               principal amount of $40,000,000.00 made by Carr Realty Square 106
               Partnership ("Square 106") and payable to the order of Lender, as
               amended by that certain First Amendment to Promissory Note dated
               April 26, 1996 between Square 106 and Lender.

          3.   Promissory Note dated April 15, 1994 in the original principal
               amount of $10,000,000.00 made by Square 106 and payable to the
               order of Lender, as amended by that certain First Amendment to
               Promissory Note dated October 12, 1995 between Square 106 and
               Lender and by that certain Second Amendment to Promissory Note
               dated April 26, 1996 between Square 106 and Lender.

          4.   Promissory Note dated October 12, 1995 in the original principal
               amount of $40,000,000 made by Borrower and payable to the order
               of Lender, as amended by that certain First Amendment
<PAGE>

               to Promissory Note dated April 26, 1996 between Borrower and
               Lender.

The Existing Notes and the respective principal amount of indebtedness evidenced
thereby and additional indebtedness of Borrower to Lender are hereby combined
and consolidated to constitute one indebtedness in the principal amount of ONE
HUNDRED EIGHTY-THREE MILLION SEVEN HUNDRED THOUSAND DOLLARS ($183,700,000.00).
The manner and timing of payment and the other terms, covenants, agreements and
provisions of the Existing Notes are hereby modified, amended and restated in
their entirety so that henceforth the terms, provisions, covenants and
agreements thereof shall be as set forth herein, and in the event of any
conflict in the terms, provisions, covenants or agreements between the Existing
Notes and this Note, this Note shall prevail.  The Existing Notes are attached
hereto and shall be negotiated only with this Note.

          For value received, Borrower promises to pay to the order of Lender,
at 720 E. Wisconsin Avenue, Milwaukee, WI  53202 or at such other place as
Lender shall designate in writing, in coin or currency which, at the time or
times of payment, is legal tender for public and private debts in the United
States, the principal sum of ONE HUNDRED EIGHTY-THREE MILLION SEVEN HUNDRED
THOUSAND DOLLARS or so much thereof as shall have been advanced from time to
time plus interest on the outstanding principal balance at the rate and payable
as follows:

               Interest shall accrue from the date of advance until maturity at
          the rate of eight and twelve hundredths percent (8.12%) per annum (the
          "Interest Rate").

               Accrued interest only on the amount advanced shall be paid in
          arrears on the first day of the month following the date hereof and on
          the first day of each month thereafter until and including the
          Amortization Period Commencement Date.  On the first day of the month
          following the Amortization Period Commencement Date, and on the first
          day of each month thereafter until maturity, installments of principal
          and interest shall be paid in an amount equal to the unpaid principal
          balance of this Note on the Amortization Period Commencement Date
          multiplied by .00779782431.  Installments shall be made directly to
          Lender by electronic transfer of funds using the Automated Clearing
          House System.  All installments shall be applied first in payment of
          interest, calculated monthly on the unpaid principal balance, and the
          remainder of each installment shall be applied in payment of
          principal.  The entire unpaid principal balance plus accrued

                                       2
<PAGE>

          interest thereon shall be due and payable on April 1, 2009 (the
          "Maturity Date").

               As used herein, "Amortization Period Commencement Date" means
          April 1, 2001.

          Borrower shall have the right, upon thirty (30) days advance written
notice, beginning on April 1, 2002 of making a partial prepayment (a "Partial
Prepayment") or a full prepayment (a "Full Prepayment") of this Note with a
prepayment fee.  A Partial Prepayment may only be made in connection with a
Partial Release (as hereinafter defined).  The prepayment fee represents
consideration to Lender for loss of yield and reinvestment costs and shall also
be payable whenever prepayment occurs as a result of a condemnation or sale
under threat of condemnation of all or substantially all of the Property.

In the event of a Full Prepayment, the fee shall be the greater of Yield
Maintenance or 1% of the outstanding principal balance of this Note.

In the event of a Partial Prepayment, the fee shall be equal to the product of
(i) a fraction, the numerator of which is the amount of principal being prepaid
and the denominator of which is the outstanding principal balance of this Note
immediately prior to such prepayment, and (ii) the prepayment fee that would be
payable if there were a Full Prepayment at such time.

As used herein, "Yield Maintenance" means the amount, if any, by which

          (i)  the present value of the Then Remaining Payments (as hereinafter
               defined) calculated using a periodic discount rate (corresponding
               to the payment frequency under this Note) which, when compounded
               for such number of payment periods in a year, equals the per
               annum effective yield of the Most Recently Auctioned United
               States Treasury Obligation having a maturity date equal to the
               Maturity Date (or, if there is no such equal maturity date, then
               the linearly interpolated per annum effective yield of the two
               Most Recently Auctioned United States Treasury Obligations having
               maturity dates most nearly equivalent to the Maturity Date) as
               reported by The Wall Street Journal five business days prior to
                           -----------------------
               the date of prepayment; exceeds

          (ii) the outstanding principal balance of this Note (exclusive of all
               accrued interest).

                                       3
<PAGE>

If such United States Treasury obligation yields shall not be reported as of
such time or the yields as of such time shall not be ascertainable, then the
periodic discount rate shall be equal to the Treasury Constant Maturity Series
yields reported, for the latest day for which such yields shall have been so
reported, as of five business days preceding the prepayment date, in Federal
Reserve Statistical Release H.15 (519) (or any comparable successor publication)
for actively traded United States Treasury obligations having a constant
maturity most nearly equivalent to the Maturity Date.

          All parties at any time liable, whether primarily or secondarily, for
payment of indebtedness evidenced hereby, for themselves, their heirs, legal
representatives, successors and assigns, respectively, expressly waive
presentment for payment, notice of dishonor, protest, notice of protest, and
diligence in collection; consent to the extension by Lender of the time of said
payments or any part thereof; further consent that the Property or any part
thereof may be released by Lender, without in any way modifying, altering,
releasing, affecting, or limiting their respective liability or the lien of the
Lien Instrument; and agree to pay reasonable attorneys' fees and expenses of
collection in case this Note is placed in the hands of an attorney for
collection or suit is brought hereon and any attorneys' fees and expenses
incurred by Lender to enforce or preserve its rights under any of the Loan
Documents (as defined in the Lien Instrument) in any bankruptcy or insolvency
proceeding.

          Any principal, interest or other amounts payable under any of the Loan
Documents, not paid when due (without regard to any notice and/or cure
provisions contained in any of the Loan Documents), including principal becoming
due by reason of acceleration by Lender of the entire unpaid balance of this
Note, shall bear interest from the due date thereof until paid at the Default
Rate.  As used herein, "Default Rate" means the lower of a rate equal to the
interest rate in effect at the time of the default as herein provided plus 5%
per annum or the maximum rate permitted by law.

          No provision of this Note shall require the payment or permit the
collection of interest, including any fees paid which are construed under
applicable law to be interest, in excess of the maximum permitted by law.  If
any such excess interest is collected or herein provided for, or shall be
adjudicated to have been collected or be so provided for herein, the provisions
of this paragraph shall govern, and Borrower shall not be obligated to pay the
amount of such interest to the extent that it is in excess of the amount
permitted by law.  Any such excess collected shall, at the option of Lender,
unless otherwise required by applicable law, be immediately refunded to Borrower
or credited on the principal of this Note immediately upon Lender's awareness of
the collection of such excess.

                                       4
<PAGE>

          Notwithstanding any provision contained herein or in the Lien
Instrument to the contrary, if Lender shall take action to enforce the
collection of the indebtedness evidenced hereby or secured by the Lien
Instrument (collectively, the "Indebtedness"), its recourse, except as provided
below, shall be limited to the Property or the proceeds from the sale of the
Property, the proceeds from the sale of the assets of Borrower, and the proceeds
realized by Lender in exercising its rights and remedies (i) under the Absolute
Assignment (as defined in the Lien Instrument), (ii) under the Guarantee of
Recourse Obligations of even date herewith executed by CarrAmerica Realty
Corporation for the benefit of Lender, (iii) under any of the other Loan
Documents (as defined in the Lien Instrument) and (iv) in any other collateral
securing the Indebtedness.  If such proceeds are insufficient to pay the
Indebtedness, Lender will never institute any action, suit, claim or demand in
law or in equity against the partners of Borrower for or on account of such
deficiency; provided, however, that the provisions contained in this paragraph

          (i)  shall not in any way affect or impair the validity or
               enforceability of the Indebtedness or the Lien Instrument; and

          (ii) shall not prevent Lender from seeking and obtaining a judgment
               against the general partner(s) of Borrower as well as Borrower,
               and the general partner(s) of Borrower as well as Borrower shall
               be personally liable, for the Recourse Obligations.

As used herein, the term "Recourse Obligations" means

          (a) Rents and other income from the Property from and after the date
          which is forty-five (45) days prior to the occurrence of an Event of
          Default, which Event of Default remains uncured prior to the
          foreclosure sale of the Property pursuant to the Lien Instrument or
          the conveyance of the Property to Lender in lieu of foreclosure, which
          rents and other income have not been applied to the payment of
          principal and interest on this Note or to reasonable operating
          expenses of the Property,

          (b) Amounts necessary to repair any damage to the Property caused by
          the intentional acts or omissions of Borrower or those acting on
          behalf of Borrower,

          (c) Insurance loss and condemnation proceeds released to Borrower but
          not applied in accordance with any agreement between Borrower and
          Lender as to their application and consistent with the terms and
          provisions of the Lien Instrument,

                                       5
<PAGE>

          (d) The amount of insurance loss proceeds which would have been
          available with respect to a casualty on the Property, but were not
          available due to the default by Borrower in carrying all insurance
          required by Lender,

          (e) Damages suffered by Lender as a result of fraud or
          misrepresentation in connection with the Indebtedness by Borrower or
          any other person or entity acting on behalf of Borrower,

          (f) Amounts necessary to pay real estate taxes, special assessments
          and insurance premiums with respect to the Property either paid by
          Lender and not reimbursed prior to, or remaining due or delinquent on,
          either (i) the later of (A) the date on which title vests in the
          purchaser at the foreclosure sale of the Property pursuant to the Lien
          Instrument or (B) the date on which Borrower's statutory right of
          redemption shall expire or be waived or (ii) the date of the
          conveyance of the Property to Lender in lieu of foreclosure, and

          (g) All outstanding amounts due under the Indebtedness, including
          principal, interest, and other charges if there shall be a violation
          of any of the provisions of the Lien Instrument following the caption
          entitled "Due on Sale".
                    -----------

          This Note, the interpretation hereof and the rights, obligations,
duties and liabilities hereunder shall be governed and controlled by the laws of
the District of Columbia.

                              CARR REALTY, L.P., a Delaware limited partnership

                              By:  CarrAmerica Realty Corporation, a
                                   Maryland corporation, general partner

                                   By:  /s/ Paul R. Adkins
                                       -----------------------------
(corporate seal)                   Name:    Paul R. Adkins
                                         ---------------------------
                                   Title:   Senior Vice President
                                          --------------------------

                              Attest:  /s/ Linda A. Madrid
                                      ------------------------------

                                       6