Boston ADU permits are reviewed under the Boston Zoning Code (BZC) Article 65 and underlying neighborhood districts, with statewide ADU-by-right standards established by the 2024 Massachusetts Affordable Homes Act (Chapter 150 of the Acts of 2024). Detached ADUs under 900 sq ft are allowed by-right in single-family zones statewide, but Boston still requires building, electrical, plumbing, and zoning permits issued by the Inspectional Services Department (ISD).
ADU permitting in Boston combines state and city review. The Massachusetts Affordable Homes Act of 2024 (Chapter 150, codified at G.L. c. 40A § 3A) allows accessory dwelling units of up to 900 square feet (or half the gross floor area of the principal dwelling, whichever is less) by-right in any single-family zoning district statewide, including Boston single-family districts. Boston's existing ADU pilot (originally launched 2017, expanded citywide 2020) operates alongside the state law. Applicants file through Boston's Inspectional Services Department for building, plumbing, electrical, and gas permits, plus a zoning review through the BZC. Lot must meet basic dimensional standards; egress, ventilation, and smoke alarms must meet the Massachusetts State Building Code 9th Edition. Historic district properties additionally need Boston Landmarks Commission review.
Constructing or occupying an ADU without ISD permits is a violation of the Massachusetts State Building Code and Boston Zoning Code, subject to stop-work orders, daily fines, and required removal. Unpermitted electrical or plumbing work voids homeowner insurance coverage. Historic district violations trigger separate Landmarks Commission enforcement.
Boston, MA
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Boston, MA
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