What is the ADU size limit in Massachusetts?

As of October 2026, a protected-use ADU can be up to 900 square feet or half the gross floor area of the main home, whichever is smaller. Towns may allow larger units locally.

When did the Massachusetts ADU law take effect?

The by-right ADU rule from the Affordable Homes Act took effect February 2, 2025. The state regulations, 760 CMR 71.00, took effect January 31, 2025.

Can a Massachusetts town require me to live on the property?

No. Under the state law, a town cannot require owner occupancy or a family relationship for a protected-use ADU or the main home. Boston, which is not covered by the law, sets its own rules.

Do I need a special permit for an ADU in Massachusetts?

Not for a protected-use ADU in a single-family district. Towns cannot require a special permit or other discretionary approval for its use or rental, though they may use site plan review and apply setback, bulk and height rules.

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What does the Massachusetts ADU law allow?

Updated October 2026

As of October 2026, Massachusetts lets an owner in a single-family zoning district build an ADU by right, without a special permit, in cities and towns governed by the state Zoning Act. The unit can be up to 900 sq ft or half the main home's gross floor area, whichever is smaller. The rule took effect February 2, 2025. Boston is not covered.

Where does the rule come from?

Section 8 of the Affordable Homes Act, Chapter 150 of the Acts of 2024, signed August 6, 2024, amended the state Zoning Act, M.G.L. c. 40A. The state's regulations, 760 CMR 71.00, took effect January 31, 2025, and the by-right rule applied from February 2, 2025.

What does the law guarantee?

By right: a town cannot require a special permit or other discretionary zoning approval for the use or rental of a protected-use ADU.
Size: up to 900 sq ft or half the gross floor area of the main home, whichever is smaller.
Entrance: a separate entrance, direct from outside or through a shared hall or corridor that meets building code egress rules.
No owner occupancy: a town cannot require the owner, or a relative, to live in either unit.
Parking: at most one space, and none within half a mile of qualifying transit. See the parking guide.

What can a town still regulate?

Towns can use site plan review and apply Title 5, setback, bulk and height rules, and they can restrict or ban short-term rentals of ADUs. Many have updated their bylaws: Brookline did so in May 2026 and Newton in April 2025. A town can also choose to allow more than the state minimum.

Small backyard home (ADU): A small cabin in the woods

How many ADUs have been approved?

Mass.gov reports 2,084 ADUs approved out of 2,824 applications, covering 2025 and the first half of 2026.

Are the regulations changing?

The state proposed revisions to 760 CMR 71.00 in 2026, with public comment from September 11 to October 9, 2026. The final text was not available when we checked in October 2026, so we confirm the current version before each design.

When does the state rule not settle the question?

It depends on your zoning district and your town's dimensional rules. The by-right guarantee applies in single-family districts; in other districts, local zoning decides. Setbacks, height and lot coverage are still local and can limit what fits. We read both layers for your parcel.

Check your lot against the law

Send your address. We confirm your zoning district and the local rules layered on top.