Building an ADU in Massachusetts

What state law lets you build

Governor Healey signed the Affordable Homes Act on August 6, 2024. Section 8 changed the state Zoning Act so that one ADU is allowed by right in single-family zoning districts. The state housing office lists the start date as Sunday, February 2, 2025.

The law says no zoning rule "shall prohibit, unreasonably restrict or require a special permit or other discretionary zoning approval" for a single ADU, or for renting it, in a single-family residential zoning district.

"Single-family residential zoning district" is broader than it sounds. The state regulation defines it as any district where single-family homes are allowed, whether by right or by special permit. So most residential districts count.

An ADU must:

• be a self-contained home with its own sleeping, cooking and bathroom facilities, on the same lot as the main home;
• have a separate entrance, either from outside or through a shared hall that meets the building code for safe exit;
• be no larger than half the main home's gross floor area or 900 square feet, whichever is smaller.

It can be inside the house (a basement apartment, for example), an addition, or a separate backyard cottage or converted garage. Your town cannot insist that it be attached or detached. [1][2][3][5][6]

How big it can be

The size cap is the smaller of 900 square feet or half the gross floor area of the main home. For a 1,400 sq ft house, that is 700 sq ft. For a 2,000 sq ft house, it is 900 sq ft.

"Gross floor area" under the state regulation counts all floors with legal ceiling height, including basements and lofts. It leaves out crawl spaces, garage parking areas, attics, enclosed porches and similar spaces. If there is more than one main home on the lot, the largest one sets the limit.

Towns may add their own, smaller size limits. But the law says no town may "unreasonably restrict" an ADU that is not a short-term rental, and the regulation spells out what counts as unreasonable (next section). See our ADU cost guide for what different sizes tend to cost. [2][6]

What your town can and can't require

Towns may still apply "reasonable regulations" to a protected ADU, such as site plan review, Title 5 septic rules, setbacks, bulk and height. But 760 CMR 71.00 sets firm limits.

Your town cannot:

• require that you, or anyone, live in the ADU or the main home as an owner;
• limit who may live in the ADU by income, age, family relationship or student status;
• cap the number of ADUs in town, or count ADUs in density limits;
• require more than one extra parking space, or any space near transit (see below);
• require a minimum lot size for the ADU;
• require a separate water, sewer or electric connection (a utility, state law or a court still can);
• require an impact study or impact fee that a new single-family home in the same district would not need.

Setbacks, lot coverage, open space, height and number of stories cannot be stricter than for the main home, a single-family home or an accessory building in the district, whichever is more permissive. Design standards must be clear and measurable and cannot be ones that would not apply to a single-family home in the district.

A town also cannot treat a modular (factory-built) ADU more strictly than the state building code does. Site plan review, if your town uses it, must be "clear and objective" and consistent with an as-of-right process.

The state housing office has proposed revisions to 760 CMR 71.00, with public comments taken from September 11 to October 9, 2026. The rules on this page are the regulation now in effect, so check the state ADU page for any update. [1][3][6]

Parking near transit

If any part of your lot is within half a mile of a transit station, your town cannot require any extra parking for the ADU. Otherwise it can require no more than one extra space.

The regulation defines a transit station as a subway station (MBTA Red, Green, Orange, Silver or Blue Line stops), a commuter rail station with year-round regular service, a ferry terminal with year-round regular service, or a bus station, meaning a place where people board a bus run by the MBTA or a regional transit authority. [1][6]

Garage conversions and older, nonconforming lots

Many lots and houses in Massachusetts do not meet today's zoning. The regulation says a town may not stop an ADU in an existing structure, main home or lot because of that nonconformity, as long as the space can be converted in line with the building code, the regulation and state law. An ADU that is nonconforming can still be a protected ADU.

That makes an existing garage, carriage house or barn a common candidate. Read our garage conversion guide for the general steps, and confirm the building code, fire code and septic requirements with your local building department and board of health.

If your lot uses a septic system, Title 5 (310 CMR 15.000) still applies, but a town cannot apply septic rules to an ADU that are stricter than for a single-family home in the district. The state housing office says to ask your local board of health about septic questions for a specific property. [3][6]

A second ADU, floodplains and historic districts

The by-right protection covers one ADU per lot. If your town chooses to allow more, the law says any additional ADU in a single-family district needs a special permit.

In a floodplain or aquifer protection overlay, a town may require a special permit for the ADU if the main home would also need one, but the permit must be based on clear, objective, non-discretionary criteria.

In a local historic district, towns may set different or stricter design and size standards for ADUs, as long as they are not unreasonable under the regulation's test.

Towns are free to be more generous than the state minimum, so check your local bylaw too. [1][6]

Renting it out

You can rent the ADU long term, and your town cannot require you to live on the property. Towns may restrict or prohibit short-term rentals (as defined in M.G.L. c. 64G) in ADUs, so check local rules before listing one on a nightly-rental site.

Every ADU must get its own address under the MassGIS address standard, and the address is reported to MassGIS and the state housing office. [1][6]

Boston is different

The City of Boston says the Affordable Homes Act changes do not apply in Boston. Boston gets its zoning power from a separate 1956 state law rather than the Zoning Act (Chapter 40A) that the ADU law amended. The same thing happened with the MBTA Communities multifamily zoning law.

In Boston, ADU rules come from the city's own zoning code. See our Boston ADU guide. [4]

Help paying for an ADU

MassHousing's ADU loan offers up to $250,000 for a detached ADU and up to $150,000 for an attached one. It mixes an interest-bearing loan with zero-interest, deferred financing. You must own and live in a single-family home, meet income limits that vary by location, and already have plans and permits in hand. You apply through a participating lender.

The Massachusetts Clean Energy Center runs the Massachusetts ADU Resource Center, with a property look-up tool and typical cost ranges.

The Massachusetts Housing Partnership runs the state's ADU Incentive Program in phases. In the first phase, any Massachusetts property where homes are allowed can get one feasibility study from a listed provider, and providers have agreed to charge you no more than $500 for it.

More ideas are in our ADU financing guide. [7][8][9]

If your town's rules don't match the state law

These protections come from state law and 760 CMR 71.00, which apply to every city and town that zones under Chapter 40A. The regulation also says a town cannot enforce a prohibited or unreasonable condition that it attached to an ADU approval before the regulation took effect.

The state housing office (EOHLC) says it cannot answer questions about specific properties. It tells property owners to contact their city or town planning and building departments. Its ADU page links the law, the regulation and model zoning.

The Attorney General's Municipal Law Unit reviews town bylaws and publishes its decisions, including ADU bylaw decisions, in its decision lookup tool. Our ADU permit process guide covers the general steps. [3][6]

Massachusetts laws to know

• Affordable Homes Act (Chapter 150 of the Acts of 2024), Sections 7 and 8

  Rewrote the state definition of an ADU (Section 7) and made one ADU a protected, by-right use in single-family zoning districts (Section 8, which took effect 180 days after the Act). [3][5]

• M.G.L. c. 40A, Section 3 (ADU paragraph)

  Bars zoning that prohibits, unreasonably restricts or requires a special permit for a single ADU in a single-family district, bans owner-occupancy rules and caps parking. [1]

• M.G.L. c. 40A, Section 1A (definition)

  Defines an ADU: self-contained, separate entrance, no larger than half the main home or 900 sq ft, whichever is smaller. [2]

• 760 CMR 71.00: Protected Use Accessory Dwelling Units

  State housing office (EOHLC) rules listing what towns may not require and what counts as an unreasonable rule, in effect since Jan. 31, 2025. [3][6]

Questions people ask