5 county-level rules, plus city-specific rules for 3 cities in Suffolk County, Massachusetts.
Verified from official government sources
ADUs are now allowed by right statewide under MA Affordable Homes Act (Ch. 150, Acts of 2024, §8, effective Feb 2, 2025). No county-level ADU authority. Each Suffolk County city must comply. Max 900 sq ft or 50% of principal dwelling. Cities cannot require owner-occupancy for protected ADUs.
M.G.L. c. 40A § 1A and § 3 (Accessory Dwelling Units), as amended by Affordable Homes Act of 2024
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. [...] An ADU must: Maintain a separate entrance, either directly from the outside or through an entry hall or corridor shared with the principal dwelling suff...
No county-level shed ordinance. Accessory structures governed by each city's zoning code and 780 CMR. Typical permit threshold: structures over 120–200 sq ft or those with electrical/plumbing. Setbacks set by each city's zoning district.
Garage conversions in Suffolk County cities require building permits. Under the MA Affordable Homes Act (2025), garage conversions are a protected ADU type allowed by right in single-family residential zones statewide.
M.G.L. c. 40A § 3 (Garage Conversions allowed as ADUs by-right) and 760 CMR 71.00 Protected Use ADU Regulations
ADUs can be inside an existing home, like converting a basement into an apartment, attached to a primary residence as an addition, or completely detached, like a cottage or converted garage in a backyard. [...] An ADU must: Maintain a separate entrance, either directly from the outside or through an entry hall or corridor shared with the principal dwelling sufficient to meet the requirements of...
Carports require building permits in all four Suffolk County municipalities under 780 CMR. Zoning classifies carports as accessory structures subject to setback, height, and lot coverage limits. Boston Zoning Article 10 typically allows carports in rear yards with 3-foot side setback. Chelsea, Revere, and Winthrop Zoning each specify setback and size limits.
Foundation-built tiny homes in Suffolk County are treated as dwellings under 780 CMR and must meet state minimum dimensions. The 2025 MA Affordable Homes Act allows ADUs up to 900 sq ft by right statewide, providing a pathway for tiny homes. Tiny homes on wheels (THOWs) are generally treated as RVs and cannot be used as permanent residences in residential zones.
3 cities in Suffolk County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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