ADU rules in Plymouth County, MA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Since February 2025 an accessory dwelling unit up to 900 square feet is allowed by right in every single-family zone across Plymouth County. State law, not town discretion, made it: no special permit can be required to build one.
The 2024 Affordable Homes Act amended MGL c.40A §3 so that no zoning bylaw may prohibit, unreasonably restrict, or require a special permit for a single accessory dwelling unit in a single-family residential district. Effective February 2, 2025, this binds Brockton, Plymouth, and every South Shore town. An ADU is a self-contained unit no larger than 900 square feet or half the principal dwelling's gross floor area, whichever is smaller, under §1A. The town still issues the building permit and applies setbacks and the state building code, but it cannot zone the unit out or demand owner-occupancy.
A town bylaw that bans an ADU, caps it below the state size, or forces a special permit is unenforceable against a conforming unit. Building without the town building permit still draws a stop-work order.
Other ordinances people look up for this city. Green dot = verified primary-source excerpt.
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