ADU rules in Barnstable County, MA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
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.
Massachusetts ADU law was rewritten by Section 8 (and related Section 142) of the Affordable Homes Act, signed by Governor Healey on August 6, 2024 as Chapter 150 of the Acts of 2024 (originally House Bill H.4977). The relevant provision amends MGL c. 40A, s.3 to add a new paragraph 11 that took effect February 2, 2025. Under that provision: (1) ADUs are allowed by right (no special permit) in any zoning district that permits single-family use; (2) the ADU may be up to 900 square feet of gross floor area or 50% of the principal dwelling, whichever is less; (3) municipalities may not require owner-occupancy of either unit; (4) municipalities may not impose minimum-lot-size, additional-parking (within 0.5 miles of an MBTA or commuter-rail station), or other unreasonable conditions; and (5) municipalities retain authority over setbacks, building height, bulk, site-plan review for ADUs over 900 sf or detached new construction, short-term-rental restrictions on ADUs, and consistency with the State Building Code (780 CMR). Each Barnstable County town has updated its zoning to comply: the Town of Barnstable amended its ADU ordinance on May 15, 2025 (Town Council Item 2025-060) to revise definitions, dimensional requirements, gross-floor-area calculations, parking, and remove owner-occupancy in conformance with the new c. 40A, s.3(11). The Cape Cod Commission, the regional land-use authority created by Chapter 716 of the Acts of 1989, drafted a model ADU bylaw to assist towns and maintains an inventory of town ADU provisions, but it does not act as a permit-issuing authority for ADUs. Cape Cod Commission DRI review under Section 13 of the Act and the Enabling Regulations is triggered only at 30 or more residential units, so individual ADUs are never DRIs. Barnstable County's role is limited to regional services (Cape Cod Commission, Department of Health & Environment, Dredge, AmeriCorps Cape Cod, etc.); it does not adopt zoning bylaws. Properties subject to MA Wetlands Protection Act (MGL c. 131, s.40), Title 5 septic (310 CMR 15.000) - critical on Cape Cod due to nitrogen-sensitive watersheds and the Cape Cod 208 Plan - and FEMA V/AE coastal flood zones may face additional state and local conservation, board-of-health, and floodplain review on top of the by-right ADU zoning entitlement.
Building or occupying an ADU without the proper municipal building permit, zoning sign-off, septic (Title 5) approval, or certificate of occupancy is a violation of the local zoning bylaw and the Massachusetts State Building Code (780 CMR). The municipal building commissioner or zoning enforcement officer in each Barnstable County town - not the County or the Cape Cod Commission - issues notices of violation, stop-work orders, and pursues fines under MGL c. 40A, s.7 (zoning enforcement) and s.40 (penalties of up to $300 per offense, each day a separate offense). Local boards of health enforce Title 5 septic compliance under 310 CMR 15.000 and may require an upgraded septic system before an ADU is occupied. Conservation commissions enforce the Wetlands Protection Act (MGL c. 131, s.40) within 100 feet of wetlands and 200 feet of perennial streams. Coastal properties in V or AE flood zones must meet 780 CMR Appendix G and ASCE 24 elevation standards. The Cape Cod Commission can take enforcement action only against a Development of Regional Impact (30+ units) or a project that triggers DRI thresholds - not against an individual ADU.
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