Accessory Structures in Boston, MA (2026)
4 verified accessory structures rules for Boston, Massachusetts, sourced directly from the municipal code and official government pages.
Verified from official government sources
ADU Rules
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.
Boston ADU (Accessory Dwelling Unit) Rules
Some RestrictionsMass. Gen. Laws ch. 40A, Sec. 3 (Accessory Dwelling Units Allowed As-of-Right in Single-Family Zoning Districts)
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 re...
ADU Owner Occupancy
The Zoning Code allows one detached and one non-detached accessory dwelling unit per lot, but conditions both on occupancy: Article 2's definitions require the property owner to be the principal resident for a non-detached ADU and a resident for a detached ADU, on top of separate size, height and rental-registry conditions.
Boston ADU Owner-Occupancy Requirement
Some RestrictionsBoston Zoning Code Art. 2, § 2-1
Accessory Dwelling Unit (non-Detached). A self-contained, non-transient dwelling unit with its own separate entrance, incorporated within the footprint of the principal residential structure where the Owner is also a principal resident. ... 1.The additional gross square footage created by the non-detached ADU shall add no more than 75% of the gross square footage of the principal unit or 1,250 ...
Shed & Carport Rules
A shed or other accessory building in Boston's rear yard is capped at 15 feet in height and must sit at least 4 feet from any side lot line under Zoning Code Article 20, Section 20-2. In single-family, two-family and rowhouse districts, all accessory buildings in one rear yard together cannot exceed the coverage percentage Table B sets for that district.
Boston Accessory Building Height & Setback
Some RestrictionsBoston Zoning Code Art. 20, § 20-2
Accessory buildings may be erected in a rear yard; provided that no such building is more than fifteen feet in height or nearer than four feet to any side lot line; and provided further, that in a S, R or H district the accessory buildings in any one rear yard shall not occupy in the aggregate a greater percentage of such rear yard than that specified in Table B of Section 13-1.
Garage Conversions
Garage conversions to ADUs are explicitly allowed under the 2024 MA Affordable Homes Act by right on single-family zoned lots, subject to building code compliance and permit requirements.
Boston Garage Conversion Rules
Some RestrictionsMass.gov Accessory Dwelling Units — ADU Requirements (incl. converted garage)
The Affordable Homes Act, Section 7 and 8, amended M.G.L. Chapter 40A Section 1A and M.G.L. Chapter 40A Section 3 to allows accessory dwelling units — or ADUs — under 900 square feet to be built by-right in single-family zoning districts. [...] ADUs can be inside an existing home, like converting a basement into an apartment, attached to a primary residence as an addition, or completely detache...