ADU rules in Barnstable

A detached backyard ADU is allowed by right. § 240-47.2C lets the Building Commissioner issue a building permit for an ADU "in a new or existing detached building accessory to and on the same lot as the principal dwelling", so brand new detached construction qualifies, not just conversion of an existing garage or shed. Only one ADU is permitted per lot and § 240-47.2C(1) says that limit "is not subject to variance". A detached ADU still has to meet the underlying district's height and setback rules and be consistent in design with the principal single-family dwelling (architectural details, roof design, building spacing and orientation, door and window location, building materials). Barnstable adopted its ADU ordinance in July 2021 and amended it 5-15-2025 by Order No. 2025-060 to conform to M.G.L. c. 40A as rewritten by the Affordable Homes Act.

An ADU inside the principal dwelling, whether a basement or attic conversion, an internal division, or an addition, is allowed by right: § 240-47.2C authorizes a building permit for an ADU "within a lawful existing or new single-family dwelling to which the ADU is accessory". The unit must keep a separate entrance, either directly from outside or through an entry hall or corridor shared with the principal dwelling that satisfies state building code egress, and if that entrance is not shared with the principal dwelling's it must be less visible from the street than the main entrance. Barnstable's older "family apartment" allowance at § 240-47.1 survives as a separate and much tighter track (family members only, recorded restriction, annual affidavit, no rental to non-family) and is not a prerequisite to the ADU route.

§ 240-47.2C(4) caps an ADU at a "maximum habitable floor area that is not larger than 1/2 of the gross floor area of the principal single-family dwelling unit (exclusive of floor area that converted to the ADU), or 900 square feet, whichever is smaller", which tracks the state floor in M.G.L. c. 40A, § 1A. Garages, unfinished attics and basements, common entries, porches and decks are left out of the floor area calculation. The Zoning Board of Appeals may allow more than 900 sq ft by special permit, but only where the ADU is in an existing structure whose gross floor area already exceeds 900 sq ft; new construction has no path above the cap. Once built, an ADU may not be enlarged beyond the section's limit. There is no ADU-specific minimum lot size; height, setbacks and coverage come from the underlying zoning district.

§ 240-47.2C(11) is explicit: "The use of land or structures for an accessory dwelling unit shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling." That sentence came in with the May 2025 amendment, which the town adopted specifically because M.G.L. c. 40A, § 3 forbids an owner-occupancy condition on a protected ADU. The town's June 2025 FAQ answers the question the same way: "Is owner occupancy required for an ADU? No. The ordinance does not require owner occupancy." Owner occupancy does still apply on Barnstable's two other accessory-unit tracks, the § 240-47.1 family apartment and the Chapter 9, Article II accessory affordable apartment program, which are easy to confuse with the ADU route.

§ 240-47.2C(6) requires ADU parking to be off street and caps the extra requirement at one space, "provided that no additional parking shall be required when an ADU is located not more than 0.5 miles from a commuter rail station, subway station, ferry terminal or bus station, as those terms are defined in the ADU regulations at 760 CMR 71.02." That waiver has real reach in Hyannis, where the Hyannis Transportation Center (regional bus, plus the seasonal CapeFLYER rail service) and the harbor ferry terminals sit. The building permit application must include a site plan identifying where the ADU's off-street parking goes. Barnstable's general schedule of off-street parking requirements at § 240-56 has no ADU line and defaults unlisted uses to the Building Commissioner, but the ADU-specific cap controls.

An ADU is a by-right accessory use cleared administratively. § 240-47.2B(1) says an ADU that conforms "shall be permitted as an accessory use to a lawful dwelling use", and § 240-47.2C has the Building Commissioner issue the building permit; site plans, floor plans and elevations go in with the application under § 240-47.2B(2). The town's June 2025 FAQ states that ADUs proceed by right "with administrative review and a building permit" and that "No neighbor notice or public hearing is required". Formal site plan review does not attach: § 240-100B exempts detached single-family and two-family dwellings and their permitted accessory structures. A Zoning Board of Appeals special permit, with abutter notice and a public hearing, is needed only to exceed 900 sq ft in an existing structure. Non-discretionary sign-offs are still real work: Board of Health Title 5 certification (or DPW sewer capacity certification) under § 240-47.2C(7), and an Old King's Highway certificate of appropriateness north of Route 6.

§ 240-47.2C(10) provides that an ADU "shall be used only as a rental, except that the property owner may reside in the ADU while renting the principal dwelling", and that "Any use of an ADU as a short-term rental, as defined by G.L. c. 64G, § 1, is prohibited", which means stays of 31 days or less are out. Either or both units may be rented at once, but if the owner moves into the ADU the principal dwelling may not be short-term rented during that period. The ban survives the statewide by-right regime because M.G.L. c. 40A, § 3 expressly allows "restrictions and prohibitions on short-term rental" on an ADU. Every rented unit in Barnstable, ADU included, must be registered with the Board of Health under Code Chapter 170 (a $250 certificate per unit, renewed each December 31, amended 12-11-2025 by Order No. 2026-064), and the town also requires an annual rental affidavit that the unit is leased consistent with the ADU ordinance. The ADU cannot be condominiumized or sold away from the house: the § 240-128 definition requires the same lot and the same ownership, and the June 2025 FAQ confirms it.