Can I build an ADU in Falmouth, Massachusetts?
Yes. Since November 2024, Falmouth's zoning bylaw allows one accessory dwelling unit per lot as of right, up to 900 sq ft or half the main home's floor area, whichever is less. You need Planning Board Site Plan Review before a building permit, one on-site parking space, and the ADU can't be used for weekly or monthly summer rentals.
| ADUs per lot (as of right) | 1 [1] |
| More than one ADU | Planning Board special permit [1] |
| Max size | 900 sq ft gross floor area or 50% of the main home, whichever is less [3] |
| Detached ADU footprint | No larger than the main home's footprint [3] |
| Parking | 1 on-site space (none within 0.5 mile of a rail, subway, ferry or bus station) [4] |
| Short-term / summer rentals | Weekly/monthly rentals prohibited (main home and ADU) [2] |
| Approval | Planning Board Site Plan Review before building permit [6] |
| Accessory building max height | 22 ft [7] |
What's allowed
Falmouth replaced its old "accessory apartment" rules at the November 18, 2024 Town Meeting. The new Section 240-9.1 allows one accessory dwelling unit (ADU) per lot as of right. A second ADU, "or rental thereof", needs a special permit from the Planning Board [1]. An ADU is a self-contained unit with its own sleeping, cooking and bathroom facilities. It can be inside the main house or in an accessory building on the lot [5].
Size and design
The ADU can be at most 900 sq ft of gross floor area or 50% of the main home's gross floor area, whichever is less. A new detached ADU's footprint can't exceed the main house's footprint [3]. It needs a separate entrance that meets the state building code. Its exterior should match the character of the main house [5]. Accessory buildings are generally capped at 22 feet tall [7].
Parking and rentals
Provide one parking space on the property. No extra space is required within 0.5 mile of a commuter rail, subway, ferry or bus station [4]. Weekly or monthly "summer rentals" are prohibited, and neither the house nor the ADU may be used as commercial accommodations [2]. The section doesn't require the owner to live on the property. State law lets towns restrict short-term rentals of ADUs, but a ban on month-long rentals may go further than state law allows, so state law may override that part of the rule.
Overlay districts
Lots in the Water Resource Protection Overlay District or the Coastal Pond Overlay District must also meet those districts' rules [3]. These can limit bedrooms because of nitrogen loading.
Approval process
Every ADU, whether in the existing house, an attached structure, or a new or existing detached building, needs Site Plan Review approval from the Planning Board before a building permit is issued [6].
The 2022 zoning bylaw PDF still shows the old accessory-apartment rules (special permit, owner-occupancy, 7,500 sq ft minimum lot). Those were replaced by the 2024 section described here.
See how Falmouth compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →
Next steps
- Confirm whether your lot is in the Water Resource Protection or Coastal Pond Overlay District, which add limits.
- Design the unit within 900 sq ft (or half the main home's floor area, if smaller) with a separate entrance and one on-site parking space.
- Apply to the Planning Board for Site Plan Review under § 240-12.2, with the plans and information listed in § 240-12.2D.
- After Site Plan Review approval, apply to the Building Department (508-495-7470) for the building permit; septic or sewer capacity will be reviewed by the Health Department.
Common questions
- Can I build an ADU in Falmouth, Massachusetts?
- Yes. Since November 2024, the zoning bylaw allows one accessory dwelling unit per lot as of right. You need Planning Board Site Plan Review before a building permit.
- How big can an ADU be in Falmouth, Massachusetts?
- Up to 900 sq ft or half the main home's floor area, whichever is less. A detached ADU's footprint can be no larger than the main home's footprint.
- Does an ADU need parking in Falmouth, Massachusetts?
- Yes, one on-site space, though none is needed within 0.5 mile of a rail, subway, ferry or bus station.
- Can I rent my ADU for the summer in Falmouth, Massachusetts?
- No. Weekly or monthly summer rentals are prohibited for both the main home and the ADU.
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
- § 240-9.1C(1)-(2) Requirements, Zoning Bylaw Section 240-9.1 Accessory Dwelling Units (approved by Town Meeting Nov. 18, 2024), p. 1
(1) Only one accessory dwelling unit shall be allowed per lot as of right. (2) A Special Permit from the Planning Board shall be required for more than one accessory dwelling unit, or rental thereof, on a single lot.
- § 240-9.1C(3) Requirements, Zoning Bylaw Section 240-9.1 Accessory Dwelling Units (approved by Town Meeting Nov. 18, 2024), p. 1
(3) Weekly/monthly rentals (so called “summer rentals”) are expressly prohibited. Neither the principal dwelling nor accessory dwelling unit shall be used as commercial accommodations at any time.
- § 240-9.1C(4)-(5) Requirements, Zoning Bylaw Section 240-9.1 Accessory Dwelling Units (approved by Town Meeting Nov. 18, 2024), p. 1
(4) The accessory dwelling unit shall be a maximum of 900 square feet of gross floor area, or 50 percent of the gross floor area of the principal dwelling, whichever is less. (5) The footprint of a new detached accessory dwelling unit cannot exceed that of the principal dwelling.
- § 240-9.1C(8) Requirements, Zoning Bylaw Section 240-9.1 Accessory Dwelling Units (approved by Town Meeting Nov. 18, 2024), p. 1
(8) Notwithstanding the requirements in § 240-14.1E, accessory dwelling units shall provide one parking space on the property. No additional parking space shall be required for an accessory dwelling unit located not more than 0.5 miles from a commuter rail station, subway station, ferry terminal or bus station.
- § 240-9.1B Definition, Zoning Bylaw Section 240-9.1 Accessory Dwelling Units (approved by Town Meeting Nov. 18, 2024), p. 1
Accessory Dwelling Unit – A self-contained housing unit, inclusive of sleeping, cooking and sanitary facilities on the same lot as a principal dwelling, is subordinate in size and accessory to the principal dwelling on the lot and is located either within the principal dwelling or within an accessory structure on the lot.
- § 240-9.1E(1) Procedures, Zoning Bylaw Section 240-9.1 Accessory Dwelling Units (approved by Town Meeting Nov. 18, 2024), p. 2
(1) An accessory dwelling unit constructed within an existing single-family dwelling, within an existing accessory structure attached thereto, or within an existing or new detached accessory structure shall, prior to the issuance of a building permit, obtain Site Plan Review approval from the Planning Board pursuant to § 240- 12.2 of the Zoning Bylaw.
- § 240-11.5B(1) Accessory Structures, Maximum Height, Zoning Bylaw, Town Code Chapter 240, Articles 1-14 (November 2022 recodification), p. 116
(1) MAXIMUM HEIGHT – Unless otherwise provided for in the Maximum Height Table above, the maximum height for accessory structures shall be 22 feet.