Massachusetts residential & ADU permit data

Massachusetts legalized accessory dwelling units statewide through the Affordable Homes Act, signed in August 2024, which amended the state Zoning Act (Chapter 40A). Once the implementing regulations (760 CMR 71.00) took effect on February 2, 2025, an ADU of up to 900 square feet — or 50% of the gross floor area of the principal dwelling, whichever is smaller — must be allowed 'by right' in any single-family zoning district in essentially every community, without a special permit or discretionary hearing.

The law deliberately strips out the most common local obstacles. Municipalities may not impose an owner-occupancy requirement, so a homeowner can rent both the main house and the ADU. They also may not require more than one additional parking space for the ADU, and none within a half-mile of transit, and they cannot route a conforming unit through a special-permit process to second-guess it. Towns retain authority over reasonable dimensional, septic and building-code standards, but not over whether to allow an ADU at all.

The Affordable Homes Act works in tandem with the MBTA Communities Act (2021), which requires the 177 cities and towns served by the MBTA to zone for multifamily housing as of right near transit. The state's Supreme Judicial Court confirmed in 2025 that the MBTA Communities law is constitutional and enforceable, removing the main legal cloud over the broader reform package and clearing the way for both programs to proceed together.

Massachusetts thus joined the small group of states — with California, Washington and Oregon — that guarantee ADUs by right statewide, and it did so more recently and more sweepingly than its New England neighbors: Connecticut let towns opt out of its as-of-right default, and Maine's mandate is narrower. The state reported a sharp rise in ADU filings after the February 2025 effective date. Igni tracks live Boston permit data from the city's ISD open-data portal, plus Cambridge, so new by-right ADUs surface as they are permitted across Greater Boston.

This is an informational summary, not legal advice; local dimensional and septic rules still apply, so confirm the current state regulation and your municipality's bylaw before relying on it.