Can I build an ADU in Wellesley, Massachusetts?
Yes. Wellesley's Zoning Bylaw allows one accessory dwelling unit (attached or detached) with any single-family home in a Single Residence District, with a building permit and Planning Department compliance certification. The ADU must be 250 to 900 sq ft and no more than half the main home's floor area, and it can't be rented for less than 30 days.
| Number allowed | 1 ADU per single-family (One-Unit) dwelling; not allowed with two-family homes or town houses [1] |
| Size | At least 250 sq ft; no more than the lesser of 900 sq ft or 50% of the main home's gross floor area [2] |
| Detached ADU location | Rear and/or side yard only, or in a building that existed before Sec. 5.13 was adopted [3] |
| Height | Can't raise the structure above the main home's pre-construction height [4] |
| Minimum rental term | 30 days; no more than one lease per 30-day period [5] |
| Approval | Building permit; Planning Department must certify compliance before permit and occupancy certificate [6] |
| Ongoing obligations | Recorded deed notice and annual certification to the Planning Department [7] |
These rules come from Wellesley's Zoning Bylaw as amended through the 2025 Annual Town Meeting; any later amendments aren't reflected, so confirm with the Building Department.
Where ADUs are allowed
In Single Residence Districts, an ADU is allowed as an accessory use to any One-Unit Dwelling as long as it complies with Section 5.13; only one is allowed per home, and none are allowed in two-family homes, town houses or other multi-unit buildings [1]. The ADU can't be owned separately from the main house or turned into a condominium.
Size, placement and design
The ADU must have 250 to 900 sq ft of gross floor area, and no more than 50% of the main home's floor area (not counting the basement, garage or unenclosed decks) [2]. It can be attached or detached, but a detached ADU may go only in the rear or side yard, or in a building that already existed when Section 5.13 was adopted [3]. Exterior changes must match the main home's style, ADU entry doors can't face a street (with an exception for older detached buildings), exterior stairs can't be visible from a public way, and the work can't make the structure taller than the main home was before [4]. The ADU must also meet the lot's normal area and yard (setback) rules.
Rental and use rules
The minimum lease term is 30 days (or any longer short-term-rental period set in the Town Bylaws), and the unit can't be leased more than once in 30 days [5]. An ADU can't be used for a home occupation.
Permits and ongoing requirements
Your building permit application must say the project includes an ADU, and the Inspector of Buildings won't issue the permit or certificate of occupancy until the Planning Department certifies compliance [6]. You must record a notice at the Registry of Deeds and file an annual certification with the Planning Department [7], and tell the Planning Department if you sell. On a nonconforming lot or building, an ADU can skip Zoning Board of Appeals review if it doesn't enlarge the footprint beyond a small entry porch [8].
Owner occupancy and state law
Section 5.13 as printed has no owner-occupancy rule or special-permit requirement, though the definitions still include "ADU Property Owner" and "Owner Unit" terms. Massachusetts law (since Feb. 2, 2025) bars towns from requiring owner occupancy or a special permit for a protected ADU, so state rules control if any local provision conflicts.
See how Wellesley compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →
Next steps
- Confirm your zoning district on the Wellesley Zoning Map and check that your home is a single-family (One-Unit) dwelling.
- Size the unit: 250 to 900 sq ft and no more than half the main home's floor area; place a detached ADU in the side or rear yard and meet normal setbacks.
- Apply for a building permit with the Building Department, stating that the project includes an ADU; the Planning Department must certify compliance with Sec. 5.13 before the permit issues.
- Record the Planning Board-approved ADU notice at the Norfolk Registry of Deeds.
- File the annual compliance certification with the Planning Department and notify it if you sell the property.
Common questions
- Can I build an ADU in Wellesley, Massachusetts?
- Yes. One accessory dwelling unit, attached or detached, is allowed with a single-family home in a Single Residence District, with a building permit and Planning Department compliance certification. It isn't allowed with two-family homes or town houses.
- How big can an ADU be in Wellesley, Massachusetts?
- It must be at least 250 sq ft and no more than the lesser of 900 sq ft or 50% of the main home's gross floor area.
- Where can a detached ADU go on my lot in Wellesley, Massachusetts?
- In the rear and/or side yard only, or in a building that existed before Sec. 5.13 was adopted.
- Can I rent my ADU on Airbnb in Wellesley, Massachusetts?
- No. It can't be rented for less than 30 days, with no more than one lease per 30-day period.
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. 2.1.A.1A Accessory Dwelling Unit, Zoning Bylaw (as amended through 2025 Annual Town Meeting), p. 28
An Accessory Dwelling Unit is allowed as an accessory use to any One-Unit Dwelling provided, and for so long as, such Accessory Dwelling Unit satisfies and complies with the provisions of Section 5.13. No more than one Accessory Dwelling Unit is allowed for any One-Unit Dwelling. Accessory Dwelling Units are not allowed in Two Family Dwellings, Town Houses, or other buildings with multiple Dwelling Units.
- Sec. 5.13.C.iv, Zoning Bylaw (as amended through 2025 Annual Town Meeting), p. 188
iv. (A) Contain at least 250 square feet of gross floor area and no more than the lesser of (i) 900 sq. ft. of gross floor area or (ii) 50% of the gross floor area of the Principal Dwelling, not including the Accessory Dwelling Unit, or basement, garage, or unenclosed deck or patio areas, and (B) otherwise meet the requirements of the definition of “Accessory Dwelling Unit” in Section 1A of chapter 40A of the General Laws.
- Sec. 5.13.C.iii, Zoning Bylaw (as amended through 2025 Annual Town Meeting), p. 188
iii. An Accessory Dwelling Unit may be detached from the Principal Dwelling, but any detached Accessory Dwelling Unit may be located only in the Rear Yard and/or Side Yard of the principal dwelling or in a building on the lot which existed prior to the adoption of this Section 5.13.
- Sec. 5.13.C.ii(D), Zoning Bylaw (as amended through 2025 Annual Town Meeting), p. 187
(D) Construction of an Accessory Dwelling Unit shall not increase the height of the structure beyond the height of the Principal Dwelling prior to construction of the Accessory Dwelling Unit.
- Sec. 5.13.D.ii, Zoning Bylaw (as amended through 2025 Annual Town Meeting), p. 188
ii. The minimum leasing term for the ADU shall be the greater of 30 days or such other period governing short term rentals which may be set forth from time to time in the Town Bylaws. The Accessory Dwelling Unit may not be leased more than once in any 30-day period.
- Sec. 5.13.E.i, Zoning Bylaw (as amended through 2025 Annual Town Meeting), p. 188
The Inspector of Buildings shall not issue a building permit for construction of such building or issue a certificate of occupancy for such building until the Planning Department, in accordance with Rules and Regulations adopted by the Planning Department, certifies that the building is in compliance with the provisions of Section 5.13.D
- Sec. 5.13.E.ii, Zoning Bylaw (as amended through 2025 Annual Town Meeting), p. 188
ii. The record owner must submit an annual certification to the Planning Department, in a form determined by the Planning Board, that the Accessory Dwelling Unit has been constructed and is owned and operated in compliance with all provisions of the Zoning Bylaw.
- Sec. 5.1.B.3 Accessory Dwelling Units, Zoning Bylaw (as amended through 2025 Annual Town Meeting), p. 132
and (ii) does not increase the footprint of the existing structures on the lot other than as necessary for the construction of a separate entrance for the ADU, provided that any entrance porch does not exceed 30 square feet nor project more than five feet from the face of the building.