Can I build an ADU in Barnstable, Massachusetts?
Under the 2021 version of Barnstable's zoning ordinance we reviewed, one ADU per lot is allowed with a building permit from the Building Commissioner, either inside the house or in a detached accessory building. It can be no larger than 900 sq ft or half the main home's habitable floor area, whichever is smaller, with up to 2 bedrooms. The ordinance also requires that the ADU be used as a year-round rental (12-month minimum), but state law may override that.
| Number of ADUs per lot | 1 (not subject to variance) [2] |
| Max size | 900 sq ft or 1/2 of the main home's habitable floor area, whichever is smaller [1] |
| Max bedrooms | 2 (more needs a ZBA special permit) [3] |
| Max occupancy | 2 persons over age 18 (more needs a ZBA special permit) [4] |
| Minimum rental term | 12 consecutive months [5] |
| Parking | All ADU parking off-street [6] |
| Approval | Building permit from the Building Commissioner [7] |
What's allowed
An ADU is a self-contained unit with its own sleeping, cooking and sanitary facilities, either inside a single-family home or in a detached accessory building on the same lot and in the same ownership [1]. Only one ADU is allowed per lot [2]. The Building Commissioner issues a building permit for it [7], after you submit site plans, floor plans and elevations. An ADU isn't allowed on a lot that already has an affordable accessory apartment (Chapter 9) or a family apartment (§ 240-47.1).
Size, bedrooms and occupancy
The ADU can be no larger than half the main home's habitable floor area or 900 sq ft, whichever is smaller [1]. Garages, unfinished attics and basements, porches and decks don't count. Up to 2 bedrooms are allowed; more bedrooms or more than 900 sq ft needs a special permit from the Zoning Board of Appeals [3]. Occupancy is capped at 2 adults (children 18 and under don't count) unless the ZBA grants a special permit [4]. An addition must match the main house's design, and a separate entrance must be less visible from the street than the main entrance.
Rental rules
The ordinance says the ADU may be used only as a rental, though the owner may live in the ADU and rent the main house. Rentals of either unit must be at least 12 consecutive months, and only one of the two units may be rented at a time [5]. That effectively requires the owner to live on the property and bars short-term rentals of either unit.
Parking and septic
All ADU parking must be off-street [6]. The Board of Health must confirm septic capacity under Title 5 for the combined bedrooms, or DPW must certify sewer capacity.
State law and how current this is
The text we reviewed is the version approved July 15, 2021, which predates Massachusetts' 2024 Affordable Homes Act; the town may have amended it since. State law, in effect since February 2, 2025, may override the owner-occupancy effect of the rental rules and limits on who may live in a protected ADU. Towns may still restrict short-term rentals of ADUs.
See how Barnstable compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →
Next steps
- Confirm with the Barnstable Building Division (508-862-4038) whether § 240-47.2 has been amended since 2021 to reflect state ADU law.
- Get Board of Health sign-off on Title 5 septic capacity for the combined bedrooms (or DPW certification of sewer capacity).
- Apply to the Building Commissioner for a building permit with site plans, floor plans and elevations.
- If you need more than 2 bedrooms, more than 900 sq ft, or more than 2 adult occupants, apply to the Zoning Board of Appeals for a special permit.
- Check historic district or Old King's Highway review if your property is in one.
Common questions
- Can I build an ADU in Barnstable, Massachusetts?
- Yes. Under the 2021 ordinance text we reviewed, one ADU per lot is allowed with a building permit from the Building Commissioner, inside the house or in a detached accessory building. The town may have amended it since, so confirm with the Building Division.
- How big can an ADU be in Barnstable, Massachusetts?
- No larger than 900 sq ft or half the main home's habitable floor area, whichever is smaller, with up to 2 bedrooms. More than 900 sq ft or more bedrooms needs a Zoning Board of Appeals special permit.
- Can I rent my ADU short-term in Barnstable, Massachusetts?
- No. The 2021 ordinance requires rentals of the ADU or main house to be at least 12 consecutive months, and only one of the two units may be rented at a time. State law may override parts of that, though towns may still restrict short-term rentals of ADUs.
- Does an ADU need its own parking in Barnstable, Massachusetts?
- All ADU parking must be off-street. The Board of Health must also confirm Title 5 septic capacity for the combined bedrooms, or DPW must certify sewer capacity.
Massachusetts' statewide ADU law
Since February 2, 2025, the Affordable Homes Act (St. 2024, c. 150, §8) has protected one accessory dwelling unit (ADU) per lot in any zoning district that allows single-family homes (M.G.L. c. 40A, §3). Your town cannot ban it, require a special permit or other discretionary approval, or "unreasonably restrict" building or renting it.
What counts as a protected ADU:
- A self-contained unit with its own sleeping, cooking and bathroom facilities, on the same lot as the main home.
- No larger than 900 sq ft or half the main home's floor area, whichever is smaller.
- It can be attached or detached, and it needs a separate entrance that meets the building code.
What towns cannot require:
- That the owner live in the main home or the ADU.
- More than 1 extra parking space, or any extra space if the lot is within 0.5 miles of a subway, commuter rail, ferry or bus station.
- A minimum lot size, caps on the number of ADUs, or limits on who may live there (age, income, relationship).
What towns can still do: use reasonable site plan review, setback, height and bulk rules, require Title 5 septic compliance, and restrict or ban short-term rentals of ADUs. A second ADU on the same lot needs a special permit. State regulations at 760 CMR 71.00 (in effect since January 31, 2025) explain which rules count as unreasonable.
What the bylaw says
- § 240-128 Definitions, Accessory Dwelling Unit (ADU), Accessory Dwelling Unit zoning ordinance (Town Council order 2021-174, approved version 07-15-2021), p. 1
ADUs shall have a maximum habitable floor area that is not larger than 1/2 of the habitable 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, unless special permit relief is granted with respect to the requirements of 240-47.2(C)(4) with regard to square footage.
- § 240-47.2(C)(1), Accessory Dwelling Unit zoning ordinance (Town Council order 2021-174, approved version 07-15-2021), p. 2
(1) No more than one (1) ADU may be created per lot. This provision is not subject to variance.
- § 240-47.2(C)(4), Accessory Dwelling Unit zoning ordinance (Town Council order 2021-174, approved version 07-15-2021), p. 3
ADUs with more than two bedrooms and/or a maximum habitable floor area greater than 900 square feet may be permitted by special permit from the Zoning Board of Appeals.
- § 240-47.2(C)(5), Accessory Dwelling Unit zoning ordinance (Town Council order 2021-174, approved version 07-15-2021), p. 3
Occupancy of the ADU shall not exceed two persons; occupancy limitations shall not apply to children ages 18 and under. Occupancy of an ADU by more than two persons over the age of 18 may be permitted by special permit from the Zoning Board of Appeals.
- § 240-47.2(C)(11), Accessory Dwelling Unit zoning ordinance (Town Council order 2021-174, approved version 07-15-2021), p. 3
An ADU shall be used only as a rental, except that the owner of the property may reside in the ADU while renting the principal dwelling. The rental period for an ADU and for a principal dwelling shall not be shorter than 12 consecutive months. Either the principal dwelling or ADU, but not both, may be rented at any given time.
- § 240-47.2(C)(7), Accessory Dwelling Unit zoning ordinance (Town Council order 2021-174, approved version 07-15-2021), p. 3
(7) All parking for the ADU shall be off street.
- § 240-47.2(C), Accessory Dwelling Unit zoning ordinance (Town Council order 2021-174, approved version 07-15-2021), p. 2
The Building Commissioner may issue a Building Permit authorizing the installation and use of an Accessory Dwelling Unit within a lawful existing or new single-family dwelling to which the ADU is accessory, or in a new or existing detached building accessory to and on the same lot as the principal dwelling subject to the following: