Where we build ADUs around Boston

What should City of Boston homeowners know first?

Boston writes its own zoning under a separate state enabling act, so the 2024 Affordable Homes Act's by-right ADU rule does not apply inside city limits. As of October 2026, that leaves three situations:

• Inside the house, citywide. Owner-occupants can create an ADU within existing space, such as a basement or attic, without exterior alterations.
• Detached, in Mattapan. Mattapan's residential zoning was amended to allow detached ADUs by right on owner-occupied lots.
• Detached, under review elsewhere. Draft zoning released July 17, 2026 would allow detached ADUs by right in three residential districts and as a conditional use in four others in Hyde Park, Roslindale and West Roxbury. Comments run through October 31, 2026, and the Zoning Commission must then act.

For the detail, read Can I build an ADU in the City of Boston?

What about the cities and towns around Boston?

Quincy, Newton, Somerville and Brookline are covered by the state law. In their single-family zones, an ADU of up to 900 sq ft, or half the main home if smaller, is allowed without a special permit, and owner occupancy cannot be required, as of October 2026. Each one still controls setbacks, height and, in some cases, short-term rentals.