Massachusetts now allows ADUs up to 900 sq ft by-right in single-family zones. Here is what the law allows — and the PE-stamped engineering a permit still needs.
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Massachusetts opened the door to accessory dwelling units statewide. As of February 2, 2025, under the Affordable Homes Act, a qualifying ADU is allowed by-right in any zoning district that permits single-family homes — no special permit and no public hearing for a unit that meets the rules.
Go beyond those limits — a second ADU, more than 900 sq ft, or a multi-family lot — and local rules apply, which may require a special permit.
By-right does not mean paperwork-free. You still need a building permit, full code compliance under 780 CMR, and a certificate of occupancy — which requires proper structural design (and often coordinated MEP work), all stamped for your permit.
We prepare permit-ready, PE-stamped plans for attached and detached ADUs, and tell you up front which design is easiest to approve in your town. See our ADU design & permitting service.
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