ADU rules in Boston, MA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
ADUs are now allowed by right statewide in Massachusetts under the 2024 Affordable Homes Act, with a maximum of 900 square feet. Boston cannot require owner-occupancy or special permits for protected ADUs.
The Affordable Homes Act (Chapter 150, Acts of 2024, Section 8), effective February 2, 2025, requires all Massachusetts municipalities to allow one ADU by right on any single-family zoned lot. Maximum size is 900 square feet or 50% of the principal dwelling's gross floor area, whichever is less. ADUs may be detached cottages, garage conversions, basement or attic conversions, or additions. Boston cannot require owner-occupancy, special permits, or minimum lot sizes for protected ADUs. Boston may still regulate setbacks, height, bulk, and site plan review. ADU short-term rental bans are permitted. Near MBTA transit stations, no parking is required for ADUs.
Building permits are still required for construction. Non-compliant ADUs face standard zoning and building code enforcement. Unpermitted construction subject to fines and required remediation.
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Side-by-side rule comparisons with other cities in Suffolk County.
See how other cities in Suffolk County handle adu rules.
See how Boston's adu rules rules stack up against other locations.
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