<PAGE>   1
                                 EXHIBIT 10.20

                       ASSIGNMENT AND ASSUMPTION AGREEMENT

         THIS AGREEMENT, made and executed as of this 15th day of December,
1999, by AMERUS PROPERTIES, INC., an Iowa corporation ("Assignor") to and in
favor of 611 FIFTH AVENUE, L.L.C., an Iowa limited liability company
("Assignee").

                              W I T N E S S E T H:

         WHEREAS, Assignor owns the real property located in Polk County, Iowa
and described on Exhibit "A" attached hereto and made a part hereof by this
reference and all buildings and improvements located thereon (the "Premises");
and

         WHEREAS, concurrently herewith, Assignor is conveying the Premises to
Assignee and in connection therewith and as a part of such sale, Assignor
desires to convey and assign to Assignee all of Assignor's right, title and
interest as Lessor in and under the occupancy lease of the Premises which are
described on Exhibit "B" attached hereto and made a part hereof by this
reference (the "Lease"), and Assignee desires to accept such Lease.

         NOW, THEREFORE, in consideration of Ten Dollars ($10.00) and other good
and valuable consideration paid by Assignee, the receipt and legal sufficiency
of which Assignor does hereby acknowledge, Assignor and Assignee hereby agree as
follows:

         1. Assignor does hereby SELL, CONVEY and ASSIGN to Assignee all of
            Assignor's right, title and interest in and under the Lease subject
            to the terms, covenants, and conditions of the Lease. Assignee
            hereby accepts said assignment and assumes all obligations and
            liabilities under the Lease accruing subsequent to the date of this
            Agreement.

         2. The Agreement hereunder is intended to and shall convey and assign
            to Assignee, all of Assignor's rights under the Lease to receive
            rental for the period from this date forward, common area
            maintenance charges, utility charges, tax reimbursements, percentage
            rental, and all other avails or sums of money payable by the tenant
            thereunder, whether accrued, or otherwise.

         3. The Agreement hereunder is intended to and shall convey and assign
            to Assignee, all of Assignor's rights under the Lease to the
            security deposits and all other depository accounts payable by the
            tenant thereunder as shown on Exhibit "B" attached hereto and made a
            part hereof by this reference.


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         4. Assignee shall indemnify and save harmless Assignor from and against
            any and all claims, actions, liability and expense in connection
            with a breach of any Lease by the landlord thereunder after the date
            hereof. In case any action or proceeding is brought against
            Assignor, its agents, employees or servants by reason of a breach of
            the Lease by the landlord thereunder after the date hereof,
            Assignee, on receiving notice thereof from Assignor, agrees to
            defend such action or proceeding by adequate counsel reasonably
            acceptable to Assignor at Assignee's expense. Assignor shall
            indemnify and save harmless Assignee from and against any and all
            claims, actions, liability and expense in connection with a breach
            of any Lease by Assignor thereunder occurring during the time
            Assignor was landlord under the Lease. In case any action or
            proceeding is brought against Assignee, its agents, employees or
            servants by reason of a breach of any Lease by Assignor thereunder
            occurring during the time Assignor was landlord under the Lease,
            Assignor, on receiving notice thereof from Assignee, agrees to
            defend such action or proceeding by adequate counsel reasonably
            acceptable to Assignee at Assignor's expense.

         5. This Assignment and Assumption Agreement may be executed in several
            counterparts, each of which shall be an original and all of which
            shall constitute but one and the same instrument.

         6. TO HAVE AND TO HOLD the foregoing unto Assignee, its successors and
            assigns forever.

    IN WITNESS WHEREOF, Assignor and Assignee have executed this instrument as
of the date first above written.

ASSIGNOR:                                   ASSIGNEE:
AMERUS PROPERTIES, INC.,                    611 FIFTH AVENUE, L.L.C.,
an Iowa corporation                         an Iowa limited liability company

By:   s/ James A. McClarnon                 By:   s/ Gerald D. Neugent
    -------------------------                   --------------------------
    James A. McClarnon                          Gerard D. Neugent, Manager
    Vice President



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                                ACKNOWLEDGEMENTS



STATE OF IOWA     )
                  ) SS.:
COUNTY OF POLK    )

         On this 15th day of December, 1999, before me, the undersigned, a
Notary Public in and for said State, personally appeared JAMES A. MCCLARNON, to
me personally known, who, being by me duly sworn, did say that he is the Vice
President of AMERUS PROPERTIES, INC., an Iowa corporation, that no seal has been
procured by the corporation; that the instrument was signed on behalf of the
corporation by authority of its Board of Directors; and that JAMES A. MCCLARNON,
as such officer, acknowledged the execution of the instrument to be the
voluntary act and deed of said corporation, by it and by him voluntarily
executed.

                                                s/ Beverly L. Hutchens
                                             ----------------------------------
                                             Beverly L. Hutchens, Notary Public
                                             in and for the State of Iowa.




STATE OF IOWA     )
                  )  SS.:
COUNTY OF POLK    )

         On this 17th day of December, 1999, before me, a Notary Public in and
for said State, personally appeared GERARD D. NEUGENT, to me personally known,
who, being by me duly sworn, did say that he is the Manager of 611 FIFTH AVENUE,
L.L.C., an Iowa limited liability company; that no seal has been procured by the
limited liability company; that said instrument was signed on behalf of the
limited liability company by authority of its Member; and that GERARD D.
NEUGENT, as such Manager, acknowledged the execution of said instrument to be
the voluntary act and deed of the limited liability company, by it and by him
voluntarily executed.

                                                s/ Kathy A. Kintner
                                             ----------------------------------
                            Printed Name:    Kathy A. Kintner, Notary Public in
                                             and for the State of Iowa.



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                                   EXHIBIT "A"



The land referred to is situated in the State of Iowa, County of Scott, and is
described as follows:


         Lot 1, Park 53 Second Addition to the City of Davenport, Scott County,
Iowa.





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                                   EXHIBIT "B"


                                SCHEDULE OF LEASE

                                DES MOINES, IOWA


<TABLE>
<CAPTION>
                                      Date
                                       of                       Security
Tenant                                Lease                     Deposits
------                                -----                     --------
<S>                                   <C>                       <C>
AmerUs Life Insurance Company         12/1/99                    $  0.00
</TABLE>




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