Can I build an ADU in Hamilton, Massachusetts?
Yes. Hamilton allows one attached or detached ADU by right in all single-family residential districts with a building permit from the Building Commissioner. It can be no larger than 50% of the main home's gross floor area or 900 square feet, whichever is smaller, needs one off-street parking space, and can't be used as a short-term rental.
| Approval | By right; building permit from the Building Commissioner [1] |
| Where allowed | All single-family residential districts; attached or detached [2] |
| Number per lot | One [4] |
| Max size | Lesser of 50% of the main home's gross floor area or 900 sq ft [4] |
| Detached ADU location | Not in the front yard without a special permit [5] |
| Parking | At least 1 off-street space (not required within 0.5 mi of commuter rail or bus station) [6] |
| Short-term rental | Prohibited in ADUs [7] |
| Large detached ADU | Over 900 sq ft up to 2,000 sq ft (max 50% of main home) [8] |
| Large ADU approval | Special permit from the Planning Board ("PB" in the use table, all districts) [9] |
Where and how ADUs are allowed
The Building Commissioner is the permit-granting authority for ADUs that meet the bylaw's minimum requirements [1]. An ADU is permitted by right in all single-family residential districts and may be attached to the house or in a separate accessory building [2]. In the Table of Use Regulations, a regular ADU is marked "Y" (by right) in the R-1A, R-1B and RA districts and most Town Center sub-districts (not Bay Road Civic).
What to submit
The building permit application must include a certified plot plan, elevations of all building facades, and written Board of Health verification that the septic system can handle the ADU under Title 5 [3].
Size and design standards
- One per lot, no larger than 50% of the main home's gross floor area or 900 sq ft, whichever is smaller [4].
- The ADU must meet the normal height, story, building coverage and yard setback rules in Section 4.1.2, and must have a foundation (no RVs).
- An attached ADU must match the house's character, and any new outside entrance goes on the side or rear.
- A detached ADU can't be in the front yard without a special permit [5].
- Porches and decks serving only the ADU are capped at 150 sq ft combined. A detached ADU needs its own house number, and no new curb cut is allowed.
- No more than one swimming pool and one detached garage on a lot with an ADU.
- The ADU can't be split off as a condominium or co-op unit.
Parking and rentals
At least one off-street parking space is required for the ADU's occupants, unless it is within 0.5 miles of a commuter rail or bus station [6]. Short-term rentals (as defined in G.L. c. 64G) are prohibited in ADUs [7]. The bylaw has no owner-occupancy requirement.
Large detached ADUs
A detached ADU over 900 sq ft, up to 2,000 sq ft and no more than 50% of the main home, counts as a "Large Accessory Dwelling Unit" [8]. The use table marks it "PB" in every district, meaning it needs a special permit from the Planning Board [9]. State ADU protections cover only units up to 900 sq ft, so this special permit is not affected by state law.
State law and historic district
Section 3.6 was revised in April 2025 and says the state's Protected Use ADU regulations (760 CMR 71) control where their definitions conflict with the bylaw's. The special permit for a front-yard detached ADU and the one-pool/one-garage cap may be tested against state law, which bars unreasonable restrictions on protected ADUs. ADUs in the Hamilton Historic District that change the exterior also need Historic District Commission review.
See how Hamilton compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →
Next steps
- Ask the Board of Health for written confirmation that your septic system has capacity for the ADU (Title 5).
- Have a certified plot plan and building elevations prepared showing setbacks, height, the parking space and the ADU's gross floor area.
- Apply to the Building Department (978-626-5250) for a building permit; the Building Commissioner approves ADUs that meet Section 3.6.
- If the lot is in the Hamilton Historic District and the exterior will change, get Historic District Commission approval first; for a detached ADU over 900 sq ft, apply to the Planning Board.
Common questions
- Can I build an ADU in Hamilton, Massachusetts?
- Yes. Hamilton allows one attached or detached ADU by right in all single-family residential districts, with a building permit from the Building Commissioner.
- How big can an ADU be in Hamilton, Massachusetts?
- No larger than 50% of the main home's gross floor area or 900 square feet, whichever is smaller. A large detached ADU over 900 sq ft, up to 2,000 sq ft, needs a Planning Board special permit.
- Does an ADU need parking in Hamilton, Massachusetts?
- Yes, at least 1 off-street space, though it isn't required within 0.5 mile of a commuter rail or bus station.
- Can I use an ADU as an Airbnb in Hamilton, Massachusetts?
- No. Short-term rental is prohibited in ADUs.
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. 3.6.2, Zoning By-Law (first adopted 1954, including amendments up to December 9, 2025 STM), p. 27
The Building Commissioner shall be the permit-granting authority for Accessory Dwelling Units provided that the application meets the minimum
- Sec. 3.6.3, Zoning By-Law (first adopted 1954, including amendments up to December 9, 2025 STM), p. 28
An Accessory Dwelling Unit shall be permitted by right in all Single- Family Residential Zoning Districts, provided that the Accessory Dwelling Unit meets the minimum requirements of this Section. Accessory Dwelling Units may be attached to the Principal Dwelling unit (hereinafter referred to as an Attached Accessory Dwelling Unit) or located in a separate accessory structure (hereinafter referred to as a Detached Accessory Dwelling Unit).
- Sec. 3.6.4(4), Zoning By-Law (first adopted 1954, including amendments up to December 9, 2025 STM), p. 28
Written verification from the Board of Health that the sewage disposal system(s) shall have sufficient capacity to accommodate the Accessory Dwelling Unit in accordance with Title 5 of the State Environmental Code and the Rules and Regulations of the Board of Health.
- Sec. 3.6.5(1)-(2), Zoning By-Law (first adopted 1954, including amendments up to December 9, 2025 STM), p. 28
Only one Accessory Dwelling Unit shall be permitted on each Lot containing a Principal Dwelling Unit. 2. An Accessory Dwelling Unit shall not exceed 50 percent of the Gross Floor Area of the Principal Dwelling Unit or 900 square feet of Gross Floor Area, whichever is smaller.
- Sec. 3.6.5(6), Zoning By-Law (first adopted 1954, including amendments up to December 9, 2025 STM), p. 29
A Detached Accessory Dwelling Unit shall not be permitted in the Front Yard (as defined under Section 11.0, Definitions) of the Lot, absent a special permit.
- Sec. 3.6.5(11), Zoning By-Law (first adopted 1954, including amendments up to December 9, 2025 STM), p. 29
At least one (1) off-street, on-site parking space shall be provided for exclusive use by occupants(s) of the Accessory Dwelling Unit or their invitees, unless the Accessory Dwelling Unit is within 0.5 miles of a commuter rail station or Bus Station.
- Sec. 3.6.5(14), Zoning By-Law (first adopted 1954, including amendments up to December 9, 2025 STM), p. 29
Short term rentals as defined in G.L. c. 64G, §1 are prohibited in Accessory Dwelling Units.
- Sec. 11.0, Definitions, Zoning By-Law (first adopted 1954, including amendments up to December 9, 2025 STM), p. 195
Large Accessory Dwelling Unit: A Detached Accessory Dwelling Unit, measured in Gross Floor Area, that is larger than 900 square feet but which shall not exceed 2,000 square-feet. In no event may the size of a Large Accessory Dwelling Unit exceed 50 percent of the Gross Floor Area of the Principal Dwelling.
- Sec. 3.1.3 Key (Table of Use Regulations), Zoning By-Law (first adopted 1954, including amendments up to December 9, 2025 STM), p. 19
Uses that may be permitted by special permit in the district, in accordance with Section 10.5, are designated by identification of the Special Permit Granting Authority (“SPGA”) with authority to permit the use in that district, which is either: ZBA Zoning Board of Appeals PB Planning Board SB Select Board