ADU Size Limit: A Massachusetts Homeowner Guide
A protected-use ADU in Massachusetts may be up to 900 square feet or 50% of the principal dwelling's gross floor area, whichever is smaller, under the statewide rule effective January 31, 2025. A 1,400-square-foot main house, for example, may limit the ADU to 700 square feet, even though the statutory ceiling is 900 square feet. […]