Can I build an ADU in Weston, Massachusetts?
Yes. One accessory dwelling unit (ADU) is allowed by right in Weston's Single Family Residence Districts with a building permit from the Building Inspector. It can be no larger than half the main home's floor area or 900 sq ft, whichever is smaller. A detached ADU must sit in the rear yard, and it can't be used as a short-term rental. If you can't meet the by-right conditions, you need Planning Board site plan approval.
| Approval (typical case) | Building permit from the Building Inspector, by right [1] |
| Allowed in | Single Family Residence Districts A-D (by-right use), if not used as a short-term rental [2] |
| Max size | 1/2 the main home's gross floor area or 900 sq ft, whichever is smaller [3] |
| Detached ADU location | Rear yard, between the rear plane of the house and the opposite lot line [4] |
| Parking | Room for 1 extra off-street space (200 sq ft) in the side or rear yard, except within 1/2 mile of a transit station [5] |
| Impervious area limit (by right) | Less than 900 sq ft added, including parking [6] |
| Ownership | House and ADU must stay in single ownership [6] |
| When site plan approval is needed | Not in rear yard, 900+ sq ft impervious, exterior stair over 6 ft rise, or septic needing tree removal/grading [8] |
| Second ADU | Special permit [9] |
Where and how ADUs are allowed
Weston's Zoning By-law lists accessory dwelling units as a by-right use in the Single Family Residence Districts (A, B, C and D), provided they are "not used for short term rentals" [2]. The Building Inspector issues a building permit for an ADU when a list of conditions in Section VI.H.1 is met [1]. An ADU is a self-contained unit with its own sleeping, cooking and sanitary facilities and a separate entrance. It can be no larger than half the main home's gross floor area or 900 sq ft, whichever is smaller [3].
By-right conditions
- A detached ADU must be in the rear yard area, between the rear plane of the house and the opposite lot line [4].
- No exterior multi-level stairs, and only code-required, dark-sky-compliant lighting.
- Unless you're within half a mile of a transit station, there must be 200 sq ft in the side or rear yard for one extra off-street parking space, without more tree removal [5].
- No septic expansion that requires tree removal or regrading, and less than 900 sq ft of new impervious area including parking. The house and ADU must stay under single ownership and can't be sold separately [6].
When Planning Board review applies
If the ADU meets everything except one or more listed items, it needs site plan approval from the Planning Board instead [7]. Those items are: septic work requiring tree removal or grading, 900+ sq ft of new impervious area, an exterior stair rising more than 6 ft, or an ADU not in the rear yard [8]. If the lot or structure is nonconforming, the Zoning Board of Appeals must also make a finding. More than one ADU on a lot needs a special permit [9].
Accessory residences
A larger unit that doesn't meet the ADU definition can be approved by special permit as an "Accessory Residence." For these, the board may require the owner to live in the main house or the unit [10].
State law
Massachusetts law has protected one ADU per lot since Feb. 2, 2025. State law may override local rules that "unreasonably restrict" ADUs, such as the rear-yard location requirement, so check with the town if a condition blocks your project.
See how Weston compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →
Next steps
- Confirm your zoning district and whether your lot is within 1/2 mile of a transit station with Inspectional Services (781-786-5040).
- Prepare a plot plan showing the ADU location (rear yard if detached), the extra parking space and the total new impervious area.
- Check septic capacity with the Board of Health, and with the Conservation Commission if work is near wetlands.
- Apply to the Building Inspector for a building permit. If you can't meet every Section VI.H.1 condition, apply to the Planning Board for site plan approval first.
- Don't list the ADU on Airbnb or similar sites: ADUs used as short-term rentals aren't allowed.
Common questions
- Can I build an ADU in Weston, Massachusetts?
- Yes. One accessory dwelling unit is allowed by right in Single Family Residence Districts with a building permit from the Building Inspector. If you can't meet the by-right conditions, you need Planning Board site plan approval.
- How big can an ADU be in Weston, Massachusetts?
- No larger than half the main home's gross floor area or 900 sq ft, whichever is smaller.
- Where can a detached ADU go in Weston, Massachusetts?
- A detached ADU must sit in the rear yard. The page also lists room for 1 extra off-street parking space, except within 1/2 mile of a transit station.
- Can I rent my ADU on Airbnb in Weston, Massachusetts?
- No. An ADU can't be used as a short-term rental. A second ADU needs a special permit.
Massachusetts' statewide ADU law
Since February 2, 2025, the Affordable Homes Act (St. 2024, c. 150, §8) has protected one accessory dwelling unit (ADU) per lot in any zoning district that allows single-family homes (M.G.L. c. 40A, §3). Your town cannot ban it, require a special permit or other discretionary approval, or "unreasonably restrict" building or renting it.
What counts as a protected ADU:
- A self-contained unit with its own sleeping, cooking and bathroom facilities, on the same lot as the main home.
- No larger than 900 sq ft or half the main home's floor area, whichever is smaller.
- It can be attached or detached, and it needs a separate entrance that meets the building code.
What towns cannot require:
- That the owner live in the main home or the ADU.
- More than 1 extra parking space, or any extra space if the lot is within 0.5 miles of a subway, commuter rail, ferry or bus station.
- A minimum lot size, caps on the number of ADUs, or limits on who may live there (age, income, relationship).
What towns can still do: use reasonable site plan review, setback, height and bulk rules, require Title 5 septic compliance, and restrict or ban short-term rentals of ADUs. A second ADU on the same lot needs a special permit. State regulations at 760 CMR 71.00 (in effect since January 31, 2025) explain which rules count as unreasonable.
What the bylaw says
- Sec. VI.H.1 Accessory Dwelling Unit, By Right, Zoning By-law (Article VIII of the By-laws), through 2025 Special Town Meeting, p. 62
1. By Right under Section V.B.1. The Building Inspector may issue a building permit for an Accessory Dwelling Unit and related construction provided that:
- Sec. V.B.1.b Single Family Residence Districts, By-Right Uses, Zoning By-law (Article VIII of the By-laws), through 2025 Special Town Meeting, p. 19
b. Accessory Dwelling Units which are not used for short term rentals as defined by MGL Chapter 64G Section 1 and subject to limitations of Section VI.H
- Sec. II Definitions, Accessory Dwelling Unit, Zoning By-law (Article VIII of the By-laws), through 2025 Special Town Meeting, p. 2
A self-contained housing unit, inclusive of sleeping, cooking and sanitary facilities on the same lot as a principal dwelling, subject to otherwise applicable dimensional and parking requirements, that: (i) maintains a separate entrance, either directly from the outside or through an entry hall or corridor shared with the principal dwelling sufficient to meet the requirements of the state building code for safe egress; (ii) is not larger in gross floor area than 1/2 the gross floor area of the principal dwelling or 900 square feet, whichever is smaller;
- Sec. VI.H.1.a, Zoning By-law (Article VIII of the By-laws), through 2025 Special Town Meeting, p. 62
a. Any detached accessory dwelling shall be located in the rear yard area between the rear plane of the primary dwelling and the opposite property line provided that the opposite property line does not intersect with the boundary of a right of way.
- Sec. VI.H.1.d, Zoning By-law (Article VIII of the By-laws), through 2025 Special Town Meeting, p. 62
d. Except for lots within ½ mile of a transit station, 200 s.f. of space exists on the side or rear yards of the lot with no additional tree removal for one additional off-street parking space to serve the Accessory Dwelling Unit in addition to the off-street parking spaces required to serve the Single Family Dwelling.
- Sec. VI.H.1.f-g, Zoning By-law (Article VIII of the By-laws), through 2025 Special Town Meeting, p. 62
f. The additional impervious area added to the site including parking is less than 900 sf. g. The primary residence and Accessory Dwelling Unit shall remain under single ownership and the ownership shall not be alienated.
- Sec. VI.H.2 Site Plan Approval, Zoning By-law (Article VIII of the By-laws), through 2025 Special Town Meeting, p. 63
2. Site Plan Approval under Section V.B.2 Site Plan Approval under Section XI by the Planning Board shall be required for any Accessory Dwelling Unit that meets all the above requirements of Section VI.H.1 except one or more of the following:
- Sec. VI.H.2.b-d, Zoning By-law (Article VIII of the By-laws), through 2025 Special Town Meeting, p. 63
b. Greater than 900sf of impervious area is being added to the site for the ADU and required parking. c. An exterior stair greater than 6’ in overall rise from grade is required to access the Accessory Dwelling Unit. d. The Accessory Dwelling Unit is not located in the rear yard.
- Sec. V.B.4.b Uses Allowed by Special Permit, Zoning By-law (Article VIII of the By-laws), through 2025 Special Town Meeting, p. 22
b. More than one Accessory Dwelling Unit which are not used for short term rentals as defined by MGL Chapter 64G Section 1.
- Sec. VI.I.3.b Accessory Residence, Zoning By-law (Article VIII of the By-laws), through 2025 Special Town Meeting, p. 63
b. The owner or owners of the property shall live either in the Single Family Dwelling or in the Accessory Apartment.