Accessory Structures in Barnstable County, MA (2026)

ADU Rules

Barnstable County does not regulate accessory dwelling units (ADUs) at the county level. Massachusetts is a home-rule state under Article 89 of the Massachusetts Constitution, and zoning authority belongs to each city or town under MGL c. 40A. As of February 2, 2025, the statewide Affordable Homes Act (Chapter 150 of the Acts of 2024, codified at MGL c. 40A, s.3, paragraph 11) preempts local ADU bans: ADUs up to 900 square feet are allowed by right in single-family zoning districts in every Barnstable County town - Barnstable, Bourne, Brewster, Chatham, Dennis, Eastham, Falmouth, Harwich, Mashpee, Orleans, Provincetown, Sandwich, Truro, Wellfleet, and Yarmouth. Towns may regulate setbacks, height, bulk, short-term-rental use of an ADU, and impose site-plan review, but cannot require owner-occupancy or a discretionary special permit for a protected ADU. The Cape Cod Commission, established under the Cape Cod Commission Act (Chapter 716 of the Acts of 1989), is the regional planning agency for Barnstable County but does not approve individual ADU applications - its Development of Regional Impact (DRI) review is triggered at 30 or more residential units, far above any single ADU.