Massachusetts ADU law 2025 explained: what's allowed and what it means in Somerville
Massachusetts ADU law 2025 in plain English: by-right ADUs up to 900 sq ft, no owner-occupancy rule, parking limits, and what it means in Somerville.
Mateus MagnoLicensed Construction Supervisor · November 2, 2026 · 4 min read
Since February 2, 2025, Massachusetts law lets you build one accessory dwelling unit by right in any district that allows single-family homes, as long as it is no bigger than half the main home or 900 sq ft, whichever is smaller. Towns can't require owner occupancy, a special permit, or any parking within 0.5 miles of transit.
The Massachusetts ADU law 2025 homeowners keep asking me about is simple at its core: you can now build one small accessory apartment on your lot by right, without asking for a special permit. Since February 2, 2025, towns can only apply reasonable rules to it, not block it. Below I walk through what the state law says, what cities can and can't require, and how it plays out here in Somerville.
The short version: the state opened the door, but your local dimensional rules, the building code and a building permit still decide what fits on your lot.
Massachusetts ADU law 2025: the summary
Per the state's ADU page, Governor Healey signed the Affordable Homes Act on August 6, 2024 (Chapter 150 of the Acts of 2024). Section 8 changed the Zoning Act so ADUs are allowed by right in single-family zoning districts. The state's rules for it, 760 CMR 71.00, took effect January 31, 2025, and the protections kicked in on February 2, 2025.
Under that page, a protected ADU must:
- Have a separate entrance, either from outside or through a shared hall that meets building code egress rules.
- Be no larger than half the gross floor area of the main home or 900 sq ft, whichever is smaller.
- Follow reasonable local rules, such as size limits or short-term rental limits, if the town adopts them.
It can be inside the house (like a basement unit), attached as an addition, or fully detached in the backyard.
What towns can and can't require
The state's ADU page and the Zoning Act, M.G.L. c. 40A §3, split the rules into two columns:
| Towns MAY apply reasonable rules on | Towns may NOT |
|---|---|
| Site plan review | Require owner occupancy of the ADU or the main house |
| Setbacks, bulk and height | Require a special permit for one ADU or its rental |
| Title 5 (septic) rules | Require more than 1 parking space for the ADU |
| Short-term rentals | Require any parking within 0.5 miles of a subway, commuter rail, ferry or bus station |
The state's ADU FAQs add more detail that matters to my clients:
- Triple-deckers count. A town can't limit ADUs to lots with single-family houses. Two- and three-family homes qualify too.
- One per lot. Only one protected ADU is allowed per lot, even if the main building has several units.
- No bedroom caps beyond what state codes require, and no minimum lot size.
- No rules on who lives there. A town can't regulate the tenant.
- Towns can't shrink the 900 sq ft limit, but they may allow bigger units.
How the 900 sq ft limit is measured
The FAQs explain that gross floor area includes basements, lofts and intermediate floors, but not crawl spaces, garages, attics or enclosed porches. Basements count only if they meet building code ceiling height, even unfinished ones. For a two- or three-family house, all units count toward the main home's size.
So on most Somerville triple-deckers, the "half the house" test is not the limit. The 900 sq ft cap is.
Parking near transit
This one is big for us. With Red Line and Green Line stations plus many bus routes, much of Somerville and Cambridge sits within half a mile of a station. Per the FAQs, a "bus station" means a regular, fixed pick-up spot, not a flag stop. If you are inside that half mile, the town can't make you add a parking space for the ADU.
What the ADU law means in Somerville
Somerville already allowed backyard units before the state law. In the Neighborhood Residence district, the Somerville Zoning Ordinance §3.1.6 permits one principal building and one accessory building per lot, and lists the Backyard Cottage as an accessory building type permitted by right.
The Backyard Cottage standards (§3.1.12) set the shape:
- Max 24 ft wide, 32 ft deep, 576 sq ft floor plate, 1.5 stories, flat or gable roof.
- 3 ft minimum side and rear setbacks, and 10 ft from the main building.
- One dwelling unit, with a 3 ft paved walkway wide enough for an ambulance cot.
- Utilities must be buried underground.
- A sprinkler system is required unless the lot is a corner lot, has a driveway to the cottage, or abuts an alley.
Note the state FAQs say dimensional rules for an ADU must be the most permissive ones that apply to the main house, a single-family house or an accessory structure in that district. And Boston is not covered by this state law at all; it has its own ADU rules, per the same FAQs.
Want to see which Somerville lots have room? Check our Somerville backyard ADU map, and browse our 3D ADU designs.
By right still means permits
The FAQs are clear that by right does not mean anything goes. Every ADU still needs a building permit and has to meet building, fire and health codes, including the state lead law. Reasonable fees and utility connection rules can still apply. The state is also revising 760 CMR 71.00; its comment period closed October 9, 2026, so I watch for updates. For cost ranges, see my ADU cost guide for Massachusetts.
Somerville is now rewriting its local rules in response to this law. See the proposed 1,750 square foot cap on backyard cottages.
Need a hand?
I help homeowners in Somerville, Cambridge, Medford and across Greater Boston figure out what ADU fits their lot, then manage the permits and the build. Book a free walkthrough and I'll send a written line-item budget, usually within 48 hours.
Frequently asked questions
What is the new ADU law in Massachusetts?
Since February 2, 2025, the Affordable Homes Act lets owners build one accessory dwelling unit by right in districts that allow single-family homes. It can be no larger than half the main home or 900 sq ft, whichever is smaller.
Do I have to live on the property to build an ADU in Massachusetts?
No. Under M.G.L. c. 40A §3, towns cannot require owner occupancy of either the ADU or the main house.
Can I build an ADU on a triple-decker lot?
Yes. The state's ADU FAQs say towns cannot limit protected ADUs to single-family lots, though only one protected ADU is allowed per lot.
Do I need parking for an ADU in Somerville?
Not if the lot is within 0.5 miles of a subway, commuter rail, ferry or bus station. Farther away, a town can require at most one space.
Thinking about an ADU? See my three backyard ADU designs and how I price work.
Mateus Magno founded Magno & Co in 2020. He is a licensed Massachusetts Construction Supervisor (CS-123658), registered Home Improvement Contractor (HIC #220484) and certified Lead-Safe Renovator Supervisor who has renovated and managed over $5 million in projects across Greater Boston.
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