Can I build an ADU in Carlisle, Massachusetts?
Yes. Carlisle allows one ADU per single-family lot, attached or detached, up to 900 sq ft or half the main home's floor area, whichever is smaller. You need Site Plan Review by the Planning Board (with a public hearing) before you can apply for a building permit, and neither unit has to be owner-occupied.
| Attached or detached | Either; inside the house or in an accessory structure [1] |
| Max size | 900 sq ft or 1/2 of the main home's gross floor area, whichever is smaller [2] |
| Owner-occupancy | Not required; both units must be in common ownership [3] |
| Approval | Planning Board Site Plan Review with abutter notice and public hearing [4] |
| Application fee | $300 ($250 ADU fee + $50 abutters list) [5] |
| Parking | Must use the existing driveway; no separate driveway opening [6] |
| Short-term rental of ADU | Allowed only if the main home is owner-occupied [7] |
What's allowed
Carlisle's Zoning Bylaw Section 5.6 implements the state's Protected Use ADU law. Any lot in a single-family district that has a main home and no existing protected ADU can add one. The ADU can be attached or detached, carved out of the existing house, or built in an accessory structure such as a garage or barn [1]. It must be a complete unit with its own living, sleeping, cooking and sanitary facilities and a separate entrance that meets the building code.
Size and dimensions
The ADU can't exceed half the gross floor area of the main home or 900 square feet, whichever is smaller [2]. It must meet the setbacks, lot coverage, height and story limits that apply to the main home or to an accessory structure in the district, whichever is more permissive [8]. Gross floor area excludes garage parking areas, attics, crawl spaces and enclosed porches.
Ownership and rentals
Neither the main home nor the ADU has to be owner-occupied, but both must stay in common ownership [3]. The ADU can be used as a short-term rental (31 days or less) only if the main home is owner-occupied; otherwise short-term rental of the ADU isn't allowed [7]. State law expressly lets towns restrict short-term rental of ADUs.
Parking and design
Parking for the ADU must be built of material consistent with the existing driveway and connect to it; a separate driveway opening isn't allowed [6]. Design guidelines (roof pitch, trim, siting) are encouraged rather than required.
Approval process
You file a Site Plan Review application with the Planning Department; the Planning Board publishes notice, notifies abutters and holds a public hearing [4]. The fee is $300 [5]. The board must hold the hearing within 65 days of a complete application. Before applying for a building permit you must record the Site Plan Decision at the Middlesex North Registry of Deeds [9]. You'll also need Board of Health sign-off on septic and well capacity, and Conservation Commission review if the work is near wetlands.
See how Carlisle compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →
Next steps
- Meet with Board of Health staff early to confirm your septic system and well can serve an additional unit (Title 5).
- Have an informal discussion with the Building Commissioner and Planning staff; get the Building Commissioner's signed statement of code and square-footage conformance (Form 3).
- Assemble the Site Plan Review package: site plan with zoning table, elevations, floor plans with gross floor area, square-footage certification (Form 2), common-ownership affidavit (Form 4), and an abutters list from the Assessor's Office.
- File the application with the Town Clerk for certification, then email it to Planning staff with the $300 check; attend the Planning Board public hearing.
- Record the Site Plan Decision at the Middlesex North Registry of Deeds, then apply to the Building Department (978-369-6689) for a building permit.
Common questions
- Can I build an ADU in Carlisle, Massachusetts?
- Yes. Carlisle allows one ADU per single-family lot, attached or detached. You need Site Plan Review by the Planning Board, with a public hearing, before you can apply for a building permit.
- How big can an ADU be in Carlisle, Massachusetts?
- An ADU can be up to 900 sq ft or half the main home's gross floor area, whichever is smaller.
- Does the owner have to live on the property with an ADU in Carlisle, Massachusetts?
- No. Neither unit has to be owner-occupied, but both units must be in common ownership.
- Can I rent my ADU on Airbnb in Carlisle, Massachusetts?
- Only if the main home is owner-occupied. Short-term rental of an ADU is allowed in Carlisle only when the owner lives in the main house.
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
- Sec. 5.6.5.1, Zoning Bylaws (May 18, 2026), p. 45
The Protected Use ADU may be attached to or detached from the Principal Dwelling. The Principal Dwelling may be altered to include a Protected Use ADU. The Protected Use ADU may be located within the Principal Dwelling or within an accessory structure on the Lot.
- Sec. 5.6.5.4, Zoning Bylaws (May 18, 2026), p. 45
The Protected Use ADU shall not be larger in Gross Floor Area than ½ the Gross Floor Area of the Principal Dwelling or 900 square feet, whichever is smaller.
- Sec. 5.6.5.7, Zoning Bylaws (May 18, 2026), p. 45
Neither the Principal Dwelling nor the Protected Use ADU is required to be owner-occupied; however, the Principal Dwelling and Protected Use ADU must be held in common ownership.
- Sec. 5.6.4.2, Zoning Bylaws (May 18, 2026), p. 44
Pursuant to the Planning Board Rules and Regulations for Site Plan Review for Protected Use ADUs, the Planning Board shall publish a legal notice, notify abutters, and hold a public hearing for the Site Plan Review application for a Protected Use ADU.
- Application Submittal Requirements, Planning Board Rules and Regulations Governing Site Plan Review for a Protected Use ADU (February 9, 2026), p. 2
Application Fee: Submit a check made payable to the Town of Carlisle for $300 (which includes the $250 ADU Application fee and the $50 Abutters List fee).
- Sec. 5.6.6.4, Zoning Bylaws (May 18, 2026), p. 46
The required parking space and any additional parking spaces for the Protected Use ADU shall be constructed of a material consistent with the existing driveway and shall have vehicular access to the driveway. A separate driveway opening for the Protected Use ADU shall not be allowed.
- Sec. 5.6.5.8, Zoning Bylaws (May 18, 2026), p. 45
If the Principal Dwelling is owner-occupied, then the Protected Use ADU may be used as a Short-term Rental, as defined in M.G.L. c. 64G, s. 1. Otherwise, a Protected Use ADU shall not be used as a Short-term Rental.
- Sec. 5.6.5.5, Zoning Bylaws (May 18, 2026), p. 45
The Protected Use ADU shall comply with the dimensional setbacks, lot coverage, open space, standards for bulk and height, and number of stories required for the Principal Dwelling, or a Single-family Residential Dwelling or accessory structure in the Zoning District in which the Protected Use ADU is located, whichever is more permissive.
- Hearing and Decision Schedule, Planning Board Rules and Regulations Governing Site Plan Review for a Protected Use ADU (February 9, 2026), p. 3
Note: Before applying for a building permit to construct the Accessory Dwelling Unit, an Applicant must record the Site Plan Decision at the Middlesex North Registry of Deeds and submit a copy of the recorded Site Plan Decision with the recording information with the application for a building permit.