ADU Rules in Massachusetts
Under the Affordable Homes Act (effective 2/2/2025)
The Affordable Homes Act, signed into law on August 6, 2024, and effective February 2, 2025, establishes statewide baseline rules for Accessory Dwelling Units (ADUs) in Massachusetts. Every single-family lot in the state now has by-right permission to build one ADU up to 900 square feet or 50% of the principal dwelling, whichever is smaller. Municipalities may adopt additional local regulations but cannot restrict the baseline rights established by the state law.
State Baseline Rules
900
Max sq ft (or 50% of home)
By Right
Approval type (no special permit needed)
10
Municipalities covered
Covered Municipalities
Arlington
State baseline + local rules
Pop. 46,308
Boston
Own zoning charter
Pop. 675,647
Brookline
State baseline + local rules
Pop. 63,191
Cambridge
State baseline + local rules
Pop. 118,403
Lowell
State baseline + local rules
Pop. 115,554
Newton
State baseline + local rules
Pop. 88,923
Salem
State baseline + local rules
Pop. 44,480
Somerville
State baseline + local rules
Pop. 81,360
Springfield
State baseline + local rules
Pop. 155,929
Worcester
State baseline + local rules
Pop. 206,518