<Page>
EXHIBIT 10.20
BOSTON PROPERTIES, INC.
EXECUTIVE SEVERANCE PLAN
1. PURPOSE. Boston Properties, Inc. (the "Company") considers it
essential to the best interests of its stockholders to foster the continuous
employment of key management personnel. The Board of Directors of the Company
(the "Board") recognizes, however, that, as is the case with many publicly held
corporations, the possibility of a Change in Control (as defined in Section 2
hereof) exists and that such possibility, and the uncertainty and questions
which it may raise among management, may result in the departure or distraction
of management personnel to the detriment of the Company and its stockholders.
Therefore, the Board has determined that the Boston Properties, Inc. Executive
Severance Plan (the "Plan") should be adopted to reinforce and encourage the
continued attention and dedication of the Senior Vice-Presidents of the Company
and the Vice Presidents with ten (10) or more years of service with the Company
(each, a "Covered Employee"; collectively, the "Covered Employees"), to their
assigned duties without distraction in the face of potentially disturbing
circumstances arising from the possibility of a Change in Control. Nothing in
this Plan shall be construed as creating an express or implied contract of
employment and, except as otherwise agreed in writing between the Covered
Employee and the Company or any of its subsidiaries or affiliates (together with
the Company, the "Employers"), the Covered Employee shall not have any right to
be retained in the employ of the Employers.
2. CHANGE IN CONTROL. For purposes of this Plan, a "Change in Control"
shall mean the occurrence of any one of the following events:
(a) any "PERSON," as such term is used in Sections 13(d) and 14(d) of
the Act (other than any of the Employers, Mortimer B. Zuckerman, Edward H.
Linde, any "AFFILIATE" or "ASSOCIATE" (as such terms are defined in Rule
12b-2 under the Act) of Mortimer B. Zuckerman or Edward H. Linde, or any
trustee, fiduciary or other person or entity holding securities under any
employee benefit plan or trust of any of the Employers), together with all
"AFFILIATES" and "ASSOCIATES" (as such terms are defined in Rule 12b-2
under the Act) of such person, shall become the "BENEFICIAL OWNER" (as such
term is defined in Rule 13d-3 under the Act), directly or indirectly, of
securities of the Company representing 25 percent or more of the combined
voting power of the Company's then outstanding securities having the right
to vote in an election of the Company's Board of Directors ("Voting
Securities") (other than as a result of an acquisition of securities
directly from the Company); provided that for purposes of determining the
"BENEFICIAL OWNERSHIP" (as such term is defined in Rule 13d-3 under the
Act) of any "GROUP" of which Mortimer B. Zuckerman, Edward H. Linde or any
of their affiliates or associates is a member (each such entity or
individual, a "Related Party"), there shall not be attributed to the
"BENEFICIAL OWNERSHIP" (as such term is defined in Rule 13d-3 under the
Act) of such group any shares beneficially owned by any Related Party; or
<Page>
(b) persons who, as of the effective date of the Company's initial
public offering of Stock, constitute the Company's Board of Directors (the
"Incumbent Directors") cease for any reason, including, without limitation,
as a result of a tender offer, proxy contest, merger or similar
transaction, to constitute at least a majority of the Board, provided that
any person becoming a director of the Company subsequent to such date shall
be considered an Incumbent Director if such person's election was approved
by or such person was nominated for election by either (A) a vote of at
least two-thirds of the Incumbent Directors or (B) a vote of at least a
majority of the Incumbent Directors who are members of a nominating
committee comprised, in the majority, of Incumbent Directors; or
(c) the stockholders of the Company shall approve (A) any
consolidation or merger of the Company where the stockholders of the
Company, immediately prior to the consolidation or merger, would not,
immediately after the consolidation or merger, "BENEFICIALLY OWN" (as such
term is defined in Rule 13d-3 under the Act), directly or indirectly,
shares representing in the aggregate 60 percent or more of the voting
shares of the corporation issuing cash or securities in the consolidation
or merger (or of its ultimate parent corporation, if any), (B) any sale,
lease, exchange or other transfer to an unrelated party (in one transaction
or a series of transactions contemplated or arranged by any party as a
single plan) of all or substantially all of the assets of the Company or
(C) any plan or proposal for the liquidation or dissolution of the Company.
Notwithstanding the foregoing, a "Change of Control" shall not be deemed to
have occurred for purposes of the foregoing clause (a) solely as the result of
an acquisition of securities by the Company which, by reducing the number of
shares of Voting Securities outstanding, increases the proportionate number of
shares of Voting Securities beneficially owned by any person (as defined in the
foregoing clause (a)) to 25 percent or more of the combined voting power of all
then outstanding Voting Securities; PROVIDED, HOWEVER, that if such person shall
thereafter become the beneficial owner of any additional shares of Voting
Securities (other than pursuant to a stock split, stock dividend, or similar
transaction or as a result of an acquisition of securities directly from the
Company), then a "CHANGE OF CONTROL" shall be deemed to have occurred for
purposes of the foregoing clause (a).
3. TERMINATING EVENT. A "Terminating Event" shall mean the termination of
employment of a Covered Employee in connection with any of the events provided
in this Section 3 occurring within twelve (12) months following a Change in
Control:
(a) termination by the Employers of the employment of the Covered
Employee with the Employers for any reason other than for Cause or the
death or disability (as determined under the Employers' then existing
long-term disability coverage) of such Covered Employee. "Cause" shall
mean, and shall be limited to, the occurrence of any one or more of the
following events:
(i) a willful act of dishonesty by the Covered Employee with
respect to any matter involving any of the Employers; or
2
<Page>
(ii) conviction of the Covered Employee of a crime involving
moral turpitude; or
(iii) the deliberate or willful failure by the Covered Employee
(other than by reason of the Covered Employee's physical or mental
illness, incapacity or disability) to substantially perform the
Covered Employee's duties with the Employers and the continuation of
such failure for a period of 30 days after delivery by the Employers
to the Covered Employee of written notice specifying the scope and
nature of such failure and their intention to terminate the Covered
Employee for Cause.
A Terminating Event shall not be deemed to have occurred pursuant to
this Section 3(a) solely as a result of the Covered Employee being an
employee of any direct or indirect successor to the business or assets of
either of the Employers, rather than continuing as an employee of the
Employers following a Change in Control. For purposes of clauses (i) and
(iii) of this Section 3(a), no act, or failure to act, on the Covered
Employee's part shall be deemed "willful" unless done, or omitted to be
done, by the Covered Employee without reasonable belief that the Covered
Employee's act, or failure to act, was in the best interest of the
Employers; or
(b) termination by the Covered Employee of the Covered Employee's
employment with the Employers for Good Reason. "Good Reason" shall mean the
occurrence of any of the following events:
(i) a substantial adverse change in the nature or scope of the
Covered Employee's responsibilities, authorities, title, powers,
functions, or duties from the responsibilities, authorities, powers,
functions, or duties exercised by the Covered Employee immediately
prior to the Change in Control; or
(ii) a reduction in the Covered Employee's annual base salary as
in effect on the date hereof or as the same may be increased from time
to time except for across-the-board salary reductions similarly
affecting all or substantially all management employees; or
(iii) the relocation of the Employers' offices at which the
Covered Employee is principally employed immediately prior to the date
of a Change in Control to a location more than thirty (30) miles from
such offices, or the requirement by the Employers for the Covered
Employee to be based anywhere other than the Employers' offices at
such location, except for required travel on the Employers' business
to an extent substantially consistent with the Covered Employee's
business travel obligations immediately prior to the Change in
Control; or
(iv) the failure by the Employers to pay to the Covered Employee
any portion of his compensation or to pay to the Covered Employee any
portion of an
3
<Page>
installment of deferred compensation under any deferred compensation
program of the Employers within fifteen (15) days of the date such
compensation is due without prior written consent of the Covered
Employee; or
(v) the failure by the Employers to obtain an effective
agreement from any successor to assume and agree to perform this
Agreement.
4. SPECIAL TERMINATION BENEFITS. In the event a Terminating Event occurs
within twelve (12) months after a Change in Control with respect to a Covered
Employee,
(a) the Employers shall pay to the Covered Employee an amount equal
to the sum of the following:
(i) two (2) times the amount of the current annual base salary
of the Covered Employee, determined prior to any reductions for
pre-tax contributions to a cash or deferred arrangement or a cafeteria
plan; and
(ii) two (2) times the amount of the average annual bonus earned
by the Covered Employee with respect to the three (3) calendar years
immediately prior to the Change in Control.
Said amount shall be paid in one lump sum payment no later than thirty-one
(31) days following the Date of Termination (as such term is defined in
Section 7(b)); and
(b) the Employers shall continue to provide health, dental and life
insurance to the Covered Employee, on the same terms and conditions as
though the Covered Employee had remained an active employee, for
twenty-four (24) months after the Terminating Event; and
(c) the Employers shall provide COBRA benefits to the Covered
Employee following the end of the period referred to in Section 4(b) above,
such benefits to be determined as though the Covered Employee's employment
had terminated at the end of such period; and
(d) the Employers shall pay to the Covered Employee all reasonable
legal and mediation fees and expenses incurred by the Covered Employee in
obtaining or enforcing any right or benefit provided by this Plan, except
in cases involving frivolous or bad faith litigation initiated by the
Covered Employee; and
(e) the Employers shall provide to the Covered Employee financial
counseling, tax preparation assistance and outplacement counseling for
twenty-four (24) months after the Terminating Event.
Notwithstanding the foregoing, the special termination benefits required by
Section 4(a) shall be offset by any amount paid or payable to the Covered
Employee by the Employers under the terms of any employment agreement or other
plan.
4
<Page>
5. ADDITIONAL BENEFITS.
(a) Anything in this Plan to the contrary notwithstanding, in the
event it shall be determined that any compensation payment or distribution
by the Employers to or for the benefit of the Covered Employee, whether
paid or payable or distributed or distributable pursuant to the terms of
this Plan or otherwise (the "Severance Payments"), would be subject to the
excise tax imposed by Section 4999 of the Internal Revenue Code of 1986, as
amended (the "Code"), or any interest or penalties are incurred by the
Covered Employee with respect to such excise tax (such excise tax, together
with any such interest and penalties, are hereinafter collectively referred
to as the "Excise Tax"), then the Covered Employee shall be entitled to
receive an additional payment (a "Gross-Up Payment") such that the net
amount retained by the Covered Employee, after deduction of any Excise Tax
on the Severance Payments, any Federal, state, and local income tax,
employment tax and Excise Tax upon the payment provided by this subsection,
and any interest and/or penalties assessed with respect to such Excise Tax
and not after the deduction of any other taxes or amounts, shall be equal
to the Severance Payments. (The Gross-Up Payment is not intended to
compensate the Covered Employee for any income taxes payable with respect
to the Severance Payments.)
(b) Subject to the provisions of Section 5(c), all determinations
required to be made under this Section 5, including whether a Gross-Up
Payment is required and the amount of such Gross-Up Payment, shall be made
by Coopers & Lybrand, L.L.P. or any other nationally recognized accounting
firm selected by the Employers (the "Accounting Firm"), which shall provide
detailed supporting calculations both to the Employers and the Covered
Employee within 15 business days of the Date of Termination, if applicable,
or at such earlier time as is reasonably requested by the Employers or the
Covered Employee. For purposes of determining the amount of the Gross-Up
Payment, the Covered Employee shall be deemed to pay federal income taxes
at the highest marginal rate of federal income taxation applicable to
individuals for the calendar year in which the Gross-Up Payment is to be
made, and state and local income taxes at the highest marginal rates of
individual taxation in the state and locality of the Covered Employee's
residence on the Date of Termination, net of the maximum reduction in
federal income taxes which could be obtained from deduction of such state
and local taxes. The initial Gross-Up Payment, if any, as determined
pursuant to this Section 5(b), shall be paid to the Covered Employee within
five days of the receipt of the Accounting Firm's determination. If the
Accounting Firm determines that no Excise Tax is payable by the Covered
Employee, the Employers shall furnish the Covered Employee with an opinion
of counsel that failure to report the Excise Tax on the Covered Employee's
applicable federal income tax return would not result in the imposition of
a negligence or similar penalty. Any determination by the Accounting Firm
shall be binding upon the Employers and the Covered Employee. As a result
of the uncertainty in the application of Section 4999 of the Code at the
time of the initial determination by the Accounting Firm hereunder, it is
possible that Gross-Up Payments which will not have been made by the
Employers should have been made (an "Underpayment"). In the event that the
Employers exhaust their remedies pursuant to Section 5(c) and the Covered
Employee thereafter is required to make a payment of any Excise Tax, the
Accounting Firm shall
5
<Page>
determine the amount of the Underpayment that has occurred, consistent with
the calculations required to be made hereunder, and any such Underpayment,
and any interest and penalties imposed on the Underpayment and required to
be paid by the Covered Employee in connection with the proceedings
described in Section 5(c), shall be promptly paid by the Employers to or
for the benefit of the Covered Employee.
(c) The Covered Employee shall notify the Employers in writing of any
claim by the Internal Revenue Service that, if successful, would require
the payment by the Employers of the Gross-Up Payment. Such notification
shall be given as soon as practicable but no later than 10 business days
after the Covered Employee knows of such claim and shall apprise the
Employers of the nature of such claim and the date on which such claim is
requested to be paid. The Covered Employee shall not pay such claim prior
to the expiration of the 30-day period following the date on which he gives
such notice to the Employers (or such shorter period ending on the date
that any payment of taxes with respect to such claim is due). If the
Employers notify the Covered Employee in writing prior to the expiration of
such period that they desire to contest such claim, provided that the
Employers have set aside adequate reserves to cover the Underpayment and
any interest and penalties thereon that may accrue, the Covered Employee
shall:
(i) give the Employers any information reasonably requested by
the Employers relating to such claim,
(ii) take such action in connection with contesting such claim
as the Employers shall reasonably request in writing from time to
time, including, without limitation, accepting legal representation
with respect to such claim by an attorney selected by the Employers,
(iii) cooperate with the Employers in good faith in order
effectively to contest such claim, and
(iv) permit the Employers to participate in any proceedings
relating to such claim; provided, however, that the Employers shall
bear and pay directly all costs and expenses (including additional
interest and penalties) incurred in connection with such contest and
shall indemnify and hold the Covered Employee harmless, on an
after-tax basis, for any Excise Tax or income tax, including interest
and penalties with respect thereto, imposed as a result of such
representation and payment of costs and expenses. Without limitation
on the foregoing provisions of this Section 5(c), the Employers shall
control all proceedings taken in connection with such contest and, at
their sole option, may pursue or forego any and all administrative
appeals, proceedings, hearings and conferences with the taxing
authority in respect of such claim and may, at their sole option,
either direct the Covered Employee to pay the tax claimed and sue for
a refund or contest the claim in any permissible manner, and the
Covered Employee agrees to prosecute such contest to a determination
before any administrative tribunal, in a court of initial jurisdiction
and in one or more appellate courts, as the Employers shall determine;
provided, however, that if the
6
<Page>
Employers direct the Covered Employee to pay such claim and sue for a
refund, the Employers shall advance the amount of such payment to the
Covered Employee on an interest-free basis and shall indemnify and
hold the Covered Employee harmless, on an after-tax basis, from any
Excise Tax or income tax, including interest or penalties with respect
thereto, imposed with respect to such advance or with respect to any
imputed income with respect to such advance; and further provided that
any extension of the statute of limitations relating to payment of
taxes for the taxable year of the Covered Employee with respect to
which such contested amount is claimed to be due is limited solely to
such contested amount. Furthermore, the Employers' control of the
contest shall be limited to issues with respect to which a Gross-Up
Payment would be payable hereunder and the Covered Employee shall be
entitled to settle or contest, as the case may be, any other issues
raised by the Internal Revenue Service or any other taxing authority.
(d) If, after the receipt by the Covered Employee of an amount
advanced by the Employers pursuant to Section 5(c), the Covered Employee
becomes entitled to receive any refund with respect to such claim, the
Covered Employee shall (subject to the Employers' complying with the
requirements of Section 5(c)) promptly pay to the Employers the amount of
such refund (together with any interest paid or credited thereon after
taxes applicable thereto). If, after the receipt by the Covered Employee of
an amount advanced by the Employers pursuant to Section 5(c), a
determination is made that the Covered Employee shall not be entitled to
any refund with respect to such claim and the Employers do not notify the
Covered Employee in writing of their intent to contest such denial of
refund prior to the expiration of 30 days after such determination, then
such advance shall be forgiven and shall not be required to be repaid and
the amount of such advance shall offset, to the extent thereof, the amount
of Gross-Up Payment required to be paid.
6. WITHHOLDING. All payments made by the Employers under this Plan shall
be net of any tax or other amounts required to be withheld by the Employers
under applicable law.
7. NOTICE AND DATE OF TERMINATION; DISPUTES; ETC.
(a) NOTICE OF TERMINATION. Within twelve (12) months after a Change
in Control, any purported termination of a Covered Employee's employment
(other than by reason of death) shall be communicated by written Notice of
Termination from the Employers to the Covered Employee or vice versa in
accordance with this Section 7. For purposes of this Plan, a "Notice of
Termination" shall mean a notice which shall indicate the specific
termination provision in this Plan relied upon and the Date of Termination.
Further, a Notice of Termination for Cause is required to include a copy of
a resolution duly adopted by the affirmative vote of not less than
two-thirds (2/3) of the entire membership of the Board at a meeting of the
Board (after reasonable notice to the Covered Employee and an opportunity
for the Covered Employee, accompanied by the Covered Employee's counsel, to
be heard before the Board) finding that, in the good faith opinion of the
Board, the termination met the criteria for Cause set forth in Section 3(a)
hereof.
7
<Page>
(b) DATE OF TERMINATION. "Date of Termination," with respect to any
purported termination of a Covered Employee's employment within twelve (12)
months after a Change in Control, shall mean the date specified in the
Notice of Termination. In the case of a termination by the Employers other
than a termination for Cause (which may be effective immediately), the Date
of Termination shall not be less than 30 days after the Notice of
Termination is given. In the case of a termination by a Covered Employee,
the Date of Termination shall not be less than 15 days from the date such
Notice of Termination is given. Notwithstanding Section 3(a) of this Plan,
in the event that a Covered Employee gives a Notice of Termination to the
Employers, the Employers may unilaterally accelerate the Date of
Termination and such acceleration shall not result in a second Terminating
Event for purposes of Section 3(a) of this Plan.
(c) NO MITIGATION. The Covered Employee is not required to seek other
employment or to attempt in any way to reduce any amounts payable to the
Covered Employee by the Employers under this Plan. Further, the amount of
any payment provided for in this Plan shall not be reduced by any
compensation earned by the Covered Employee as the result of employment by
another employer, by retirement benefits, by offset against any amount
claimed to be owed by the Covered Employee to the Employers, or otherwise.
(d) MEDIATION OF DISPUTES. The parties shall endeavor in good faith
to settle within 90 days any controversy or claim arising out of or
relating to this Plan or the breach thereof through mediation with JAMS,
Endispute or similar organizations. If the controversy or claim is not
resolved within 90 days, the parties shall be free to pursue other legal
remedies in law or equity.
8. BENEFITS AND BURDENS. This Plan shall inure to the benefit of and be
binding upon the Employers and the Covered Employees, their respective
successors, executors, administrators, heirs and permitted assigns. In the event
of a Covered Employee's death after a Terminating Event but prior to the
completion by the Employers of all payments due him under this Plan, the
Employers shall continue such payments to the Covered Employee's beneficiary
designated in writing to the Employers prior to his death (or to his estate, if
the Covered Employee fails to make such designation).
9. ENFORCEABILITY. If any portion or provision of this Plan shall to any
extent be declared illegal or unenforceable by a court of competent
jurisdiction, then the remainder of this Plan, or the application of such
portion or provision in circumstances other than those as to which it is so
declared illegal or unenforceable, shall not be affected thereby, and each
portion and provision of this Plan shall be valid and enforceable to the fullest
extent permitted by law.
10. WAIVER. No waiver of any provision hereof shall be effective unless
made in writing and signed by the waiving party. The failure of any party to
require the performance of any term or obligation of this Plan, or the waiver by
any party of any breach of this Plan, shall not prevent any subsequent
enforcement of such term or obligation or be deemed a waiver of any subsequent
breach.
8
<Page>
11. NOTICES. Any notices, requests, demands, and other communications
provided for by this Plan shall be sufficient if in writing and delivered in
person or sent by registered or certified mail, postage prepaid, to a Covered
Employee at the last address the Covered Employee has filed in writing with the
Employers, or to the Employers at their main office, attention of the Board of
Directors.
12. EFFECT ON OTHER PLANS. Nothing in this Plan shall be construed to
limit the rights of the Covered Employees under the Employers' benefit plans,
programs or policies.
13. AMENDMENT OR TERMINATION OF PLAN. The Company may amend or terminate
this Plan at any time or from time to time; provided, however, that no such
amendment shall, without the consent of the Covered Employees, in any material
adverse way affect the rights of the Covered Employees, and no termination shall
be made without the written consent of the Covered Employees.
14. GOVERNING LAW. This Plan shall be construed under and be governed in
all respects by the laws of the Commonwealth of Massachusetts.
15. OBLIGATIONS OF SUCCESSORS. In addition to any obligations imposed by
law upon any successor to the Employers, the Employers will use their best
efforts to require any successor (whether direct or indirect, by purchase,
merger, consolidation or otherwise) to all or substantially all of the business
or assets of the Employers to expressly assume and agree to perform this Plan in
the same manner and to the same extent that the Employers would be required to
perform if no such succession had taken place.
Adopted: As of July 30, 1998
9