Can I build an ADU in Mashpee, Massachusetts?
Yes. In Mashpee's R-3 and R-5 residential districts, one accessory dwelling unit per lot gets a building permit (no special permit) if it meets the bylaw's criteria: no bigger than half the main home's floor area or 900 sq ft, whichever is smaller, and enough septic capacity for the combined bedrooms. The ADU and the main house may only be rented for 30 or more consecutive days at a time.
| Approval | Building permit, granted if the criteria are met (one ADU per lot) [1] |
| Districts | Allowed by right (Y) in R-3 and R-5 [2] |
| Max size | 50% of the home's gross floor area or 900 sq ft, whichever is smaller [3] |
| Wastewater | Must meet septic requirements for the combined bedrooms [4] |
| Parking | 1 off-street space if more than 0.5 mile from a transit station [6] |
| Minimum rental period (ADU and main house) | 30 consecutive days; no weekly or daily rentals [8] |
| Fine for short-term renting | $300 per day [8] |
| Certificate of Occupancy fee | $50 [10] |
Where and how
Mashpee's ADU rules are in zoning §174-45.4, amended at the May 5, 2025 Town Meeting (Attorney General approval 11-4-2025). A building permit for one ADU per lot "shall be granted" when the criteria are met [1]. The use table marks ADUs "Y" (allowed by right) in the R-3 and R-5 residential districts and "--" (not allowed) in the C-1, C-2, C-3 and I-1 commercial and industrial districts [2], the same pattern as for single-family homes. The Building Commissioner (also the Zoning Official) issues the permit.
Size and design
The ADU can be inside the house or in a new or existing detached building. Its floor area can't exceed 50% of the home's gross floor area or 900 sq ft, whichever is smaller [3]. It must be self-contained, with its own sleeping, cooking and bathroom facilities, and have a separate entrance that meets the building code. The house and ADU together must meet wastewater (septic) requirements for the combined number of bedrooms, plus Conservation Commission and historic requirements where they apply [4]. A detached ADU counts toward the lot coverage limit and must meet the §174-31 setbacks [5]; landscaping buffers may be required.
Parking and driveways
One off-street space is required if the ADU is more than 0.5 mile from a transit station [6]. Normally no new driveway or curb cut is allowed for the ADU.
Occupancy and renting
The ADU can't be occupied until the Building Inspector issues a Certificate of Occupancy, and can't be rented until you register with the Board of Health and get a Rental Certificate [7]. Both the ADU and the main house may only be rented for at least 30 consecutive days at a time, with a $300-per-day fine for weekly or daily rentals [8]. State law lets towns restrict short-term rentals of ADUs. There is no owner-occupancy requirement in the bylaw.
Other rules
The ADU can't be sold separately or turned into a condominium [9]. An amnesty lets owners of existing unpermitted ADUs apply to legalize them if the septic system complies. The Certificate of Occupancy costs $50 [10].
See how Mashpee compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →
Next steps
- Ask the Board of Health whether your septic system (and lot size, if you're in a Zone II or on a private well) supports the combined number of bedrooms.
- If the lot is near wetlands or in the Historic District, check with the Conservation Commission or Historic District Commission first.
- Apply for a building permit through PermitEyes with plans and a certified site plan; call the Building Department (508-539-1406) with questions.
- Before anyone moves in, get the Certificate of Occupancy; before renting, register with the Board of Health for a Rental Certificate (leases of 30+ days only).
Common questions
- Can I build an ADU in Mashpee, Massachusetts?
- Yes. In the R-3 and R-5 residential districts, one accessory dwelling unit per lot gets a building permit, with no special permit, if it meets the zoning bylaw's criteria, including enough septic capacity for the combined bedrooms. ADUs aren't allowed in the commercial and industrial districts.
- How big can an ADU be in Mashpee, MA?
- No more than 50% of the main home's gross floor area or 900 sq ft, whichever is smaller. It can be inside the house or detached; a detached ADU counts toward lot coverage and must meet the §174-31 setbacks.
- Can I rent my ADU on Airbnb in Mashpee, Massachusetts?
- No. On a lot with an ADU, both the ADU and the main house may only be rented for at least 30 consecutive days at a time; weekly or daily rentals carry a $300-per-day fine. You also need a Board of Health Rental Certificate before renting.
- Do I have to live on the property to have an ADU in Mashpee, Massachusetts?
- No. Mashpee's ADU bylaw has no owner-occupancy requirement. The ADU can't be sold separately or turned into a condominium.
- Does an ADU need parking in Mashpee, Massachusetts?
- One off-street parking space is required if the ADU is more than 0.5 mile from a transit station, and normally no new driveway or curb cut is allowed for 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
- §174-45.4 Accessory Dwelling Unit, Zoning Bylaws of the Town of Mashpee, Chapter 174 (2026 edition, updated as of the May 4, 2026 Annual Town Meeting), p. 86
A Building Permit authorizing one (1) accessory dwelling unit per lot shall be granted provided that the following criteria have been satisfied:
- §174-25 Table of Use Regulations, A(8) (columns R-3, R-5, C-1, C-2, C-3, I-1), Zoning Bylaws of the Town of Mashpee, Chapter 174 (2026 edition, updated as of the May 4, 2026 Annual Town Meeting), p. 33
(8) Accessory dwelling unit, subject to the Y Y -- -- -- -- provisions of §174-45.4.
- §174-45.4 B. Unit Size, Zoning Bylaws of the Town of Mashpee, Chapter 174 (2026 edition, updated as of the May 4, 2026 Annual Town Meeting), p. 86
The gross floor area of the accessory dwelling unit shall not be more than fifty percent (50%) of the gross floor area of said structure or nine hundred (900’) square feet, whichever is smaller, on the date the Building Permit application is filed.
- §174-45.4 A., Zoning Bylaws of the Town of Mashpee, Chapter 174 (2026 edition, updated as of the May 4, 2026 Annual Town Meeting), p. 86
The principal dwelling unit and accessory dwelling unit shall meet all wastewater treatment requirements for the combined number of bedrooms. The principal dwelling unit and accessory dwelling unit shall meet all Conservation Commission and historic requirements if applicable.
- §174-45.4 D. Exterior Design, Zoning Bylaws of the Town of Mashpee, Chapter 174 (2026 edition, updated as of the May 4, 2026 Annual Town Meeting), p. 87
Detached accessory dwelling units shall be consistent with the principal structure’s dominant design character, will contribute to the subject property’s lot coverage maximum and shall comply with the dimensional criteria established in §174-31.
- §174-45.4 E. Parking, Zoning Bylaws of the Town of Mashpee, Chapter 174 (2026 edition, updated as of the May 4, 2026 Annual Town Meeting), p. 87
Notwithstanding the provision of §174-39, one (1) off-street parking space shall be provided for accessory dwelling units more than 0.5 miles from a transit station. All parking for accessory dwelling units shall be off-street.
- §174-45.4 G., Zoning Bylaws of the Town of Mashpee, Chapter 174 (2026 edition, updated as of the May 4, 2026 Annual Town Meeting), p. 87
The accessory dwelling unit shall not be occupied until a Certificate of Occupancy has been issued by the Building Inspector. The accessory dwelling unit shall, further, not be rented until any applicable Rental Property Bylaw requirements, including registration with the Board of Health and the issuance of a Rental Certificate have first been met.
- §174-45.4 H., Zoning Bylaws of the Town of Mashpee, Chapter 174 (2026 edition, updated as of the May 4, 2026 Annual Town Meeting), p. 87
An accessory dwelling unit and principal dwelling to which it is accessory may be rented for periods of at least 30 consecutive days at a time and are prohibited from any use as rental units on a weekly or daily basis. Property owners who fail to comply with this requirement shall be subject to a three- hundred dollar ($300) fine each day that this violation persists.
- §174-45.4 I., Zoning Bylaws of the Town of Mashpee, Chapter 174 (2026 edition, updated as of the May 4, 2026 Annual Town Meeting), p. 87
An accessory dwelling unit is not intended for sale. The principal dwelling and accessory dwelling unit and lot on which they are located shall remain in common or single ownership, and shall not be severed in ownership, including that the lot or buildings thereon shall not be placed in a condominium form of ownership.
- Building Permit Fees (effective 2/11/2026), Building Permit Fees (effective 2/11/2026), p. 1
Occupancy Certificate $50