Boston, MA Accessory Structures: ADU Rules (2026)
ADU rules in Boston, MA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Max Size
- 900 sq ft or 50% of home
- Approval
- By right statewide
- Owner-Occupancy
- Cannot be required
- Transit Parking
- None within 0.5 mi
Summary
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.
No zoning ordinance or by-law shall prohibit, unreasonably restrict or require a special permit or other discretionary zoning approval for the use of land or structures for a single accessory dwelling unit, or the rental thereof, in a single-family residential zoning district; provided, that the use of land or structures for such accessory dwelling unit under this paragraph may be subject to reasonable regulations, including, but not limited to, 310 CMR 15.000 et seq., if applicable, site plan review, regulations concerning dimensional setbacks and the bulk and height of structures and may be subject to restrictions and prohibitions on short-term rental, as defined in section 1 of chapter 64G.
Full Breakdown
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.
Violations & Fines
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.
Frequently Asked Questions
Can I build an ADU on my property in Boston?
What are the size limits for an accessory dwelling unit in Boston?
Sources & Official References
Other rules in Boston
How Boston compares: Easiest Cities to Build an ADU·Compare Boston to another location·View the Massachusetts accessory structures overview
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ADU Rules in Nearby Cities
How other cities in this county handle adu rules.