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Can I build an ADU in Cohasset, Massachusetts?

Cohasset, Norfolk County · Bylaws checked 2026-10-03 · Reviewed · 9 cited sections

Allowed with a permit

Under the December 2024 zoning amendment, Cohasset allows one ADU per lot by right. It can be no bigger than 900 sq ft or half the main home's floor area, whichever is smaller, needs one extra off-street parking space (none near transit), and can't be used as a short-term rental. A larger ADU of up to 1,250 sq ft needs a special permit from the Zoning Board of Appeals, and the owner must live on the property.

Number allowedOne ADU per lot, by right [3]
Max size (standard ADU)900 sq ft or 1/2 of the main home's gross floor area, whichever is smaller [1]
Max size (large ADU, ADU-L)Over 900 sq ft up to 1,250 sq ft, by ZBA special permit [2]
Owner-occupancyRemoved for standard ADUs; required for an ADU-L [8]
ParkingOne extra off-street space for the ADU; none required within 0.5 miles of a commuter rail, subway, ferry or bus station [5]
Short-term rentalNot allowed in an ADU or ADU-L [6]

What the bylaw allows

The December 9, 2024 Special Town Meeting article rewrote Cohasset's ADU rules to match the state Affordable Homes Act [9]. An ADU is a self-contained unit with its own sleeping, cooking and sanitary facilities on the same lot as the main home. It needs a separate entrance and can be no bigger than half the main home's gross floor area or 900 square feet, whichever is smaller [1]. Section 4.3.B(1) now allows one ADU by right [3]. Only one ADU is allowed per lot, and none on a lot with more than one principal dwelling [4]. The ADU can be inside the main home or in an accessory building on the same lot. The lot and any additions must meet the district's dimensional rules, and exterior changes must keep the look of a residential dwelling, with any added stairways at the side or rear.

Large ADUs (ADU-L)

A unit over 900 square feet, up to 1,250 square feet, is an ADU-L [2]. It needs a special permit from the Zoning Board of Appeals and must be inside the single-family home or in an accessory building on the same lot [7]. The owner must live in either the main home or the ADU-L [8]. The main home must have at least 1,250 square feet of net floor area and be at least 10 years old. An ADU-L needs one off-street parking space on top of the main home's parking.

Parking and rentals

A standard ADU needs one off-street parking space in addition to the main home's parking. No extra space is needed if the ADU is within 0.5 miles of a commuter rail station, subway station, ferry terminal or bus station [5]. Neither an ADU nor an ADU-L may be used as a short-term rental [6].

About this source

Our copy is the amendment as printed in the Town Meeting warrant, which shows deleted words struck through. In the original, the per-bedroom parking wording and the old owner-occupancy, special-permit, 25% size and 10-per-year limits are all struck out. Plain-text copies run deleted and new words together, so check the current Chapter 300 on eCode360 with the Building Department.

See how Cohasset compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →

Next steps

  1. Confirm the current text of Zoning Bylaw §4.3.B(1) with the Building & Inspections Department (781-383-4100 ext. 5100), since our copy is the 2024 warrant markup rather than the codified bylaw.
  2. For a standard ADU (900 sq ft or less), apply for a building permit with floor plans showing gross floor area, the separate entrance and off-street parking.
  3. For a unit over 900 sq ft (ADU-L), apply to the Zoning Board of Appeals for a special permit before seeking a building permit.
  4. If you have a septic system, confirm with the Board of Public Health that it can handle the added unit.

Common questions

Can I build an ADU in Cohasset, Massachusetts?
Yes. Under the December 2024 zoning amendment, Cohasset allows one ADU per lot by right. Confirm the current text with the Building & Inspections Department, since our copy is the warrant markup rather than the codified bylaw.
How big can an ADU be in Cohasset, Massachusetts?
No bigger than 900 sq ft or half the main home's floor area, whichever is smaller. A larger ADU of up to 1,250 sq ft needs a special permit from the Zoning Board of Appeals, and the owner must live on the property.
Does an ADU need extra parking in Cohasset, Massachusetts?
Yes, one extra off-street space, but none is required within 0.5 miles of a commuter rail station, subway station, ferry terminal or bus station.
Can I rent my ADU on Airbnb in Cohasset, Massachusetts?
No. Neither an ADU nor a large ADU may be used as a short-term rental.

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.

State sources: M.G.L. c. 40A, §3 (protected ADU paragraph) · M.G.L. c. 40A, §1A (ADU definition) · EOHLC, Accessory Dwelling Units (law, FAQ, 2026 draft regulations) · 760 CMR 71.00, Protected Use Accessory Dwelling Units

What the bylaw says

Exact text from Cohasset's published bylaws. Section numbers are as printed in the source.

  1. Art. 2, Definitions, Accessory Dwelling Unit (ADU), PB-6: Amendments to Zoning Art. 4.3 Accessory Use Regulations and Art. 2 Definitions (ADUs, Affordable Homes Act), Dec. 9, 2024 STM, p. 1
    (ii) is not larger in gross floor area than ½ the gross floor area of the principal dwelling or 900 square feet, whichever is smaller; and (iii) is subject to the additional restrictions as may be imposed by Cohasset, including, but not limited to, additional size restrictions and restrictions or prohibitions on short-term rental, as defined in section 1 of G.L. chapter 64G;
  2. Art. 2, Definitions, Accessory Dwelling Unit – Large (ADU-L), PB-6: Amendments to Zoning Art. 4.3 Accessory Use Regulations and Art. 2 Definitions (ADUs, Affordable Homes Act), Dec. 9, 2024 STM, p. 1
    Accessory Dwelling Unit – Large (ADU-L): An Accessory Dwelling Unit as defined above, except the unit may be larger than 900 square feet but may not exceed 1,250 square feet.
  3. §4.3.B(1) Accessory dwelling unit (ADU), PB-6: Amendments to Zoning Art. 4.3 Accessory Use Regulations and Art. 2 Definitions (ADUs, Affordable Homes Act), Dec. 9, 2024 STM, p. 1
    (1) Accessory dwelling unit (ADU). One ADU is allowed by-right
  4. §4.3.B(1)(a)-(b), PB-6: Amendments to Zoning Art. 4.3 Accessory Use Regulations and Art. 2 Definitions (ADUs, Affordable Homes Act), Dec. 9, 2024 STM, p. 1
    (b) (a) No more than one ADU may be permitted on a lot. (c) (b) An ADU will not be permitted on a lot on which more than one principal dwelling is located.
  5. §4.3.B(1)(h) Parking, PB-6: Amendments to Zoning Art. 4.3 Accessory Use Regulations and Art. 2 Definitions (ADUs, Affordable Homes Act), Dec. 9, 2024 STM, p. 2
    except that no additional parking space shall be required for an ADU located not more than 0.5 miles from a commuter rail station, subway station, ferry terminal or bus station.
  6. §4.3.B(1)(i) Short-term rental, PB-6: Amendments to Zoning Art. 4.3 Accessory Use Regulations and Art. 2 Definitions (ADUs, Affordable Homes Act), Dec. 9, 2024 STM, p. 2
    (i) No ADU shall be used as a short-term rental as such term is defined by G.L. c. 64G.
  7. §4.3.B(1.1) Accessory Dwelling Unit - Large, PB-6: Amendments to Zoning Art. 4.3 Accessory Use Regulations and Art. 2 Definitions (ADUs, Affordable Homes Act), Dec. 9, 2024 STM, p. 3
    (1.1) An Accessory Dwelling Unit- Large (ADU-L) may be allowed by special permit granted by the Zoning Board of Appeals within a single-family detached dwelling located in a single-family residential district or within an accessory building on the same lot as such dwelling, subject to the following requirements:
  8. §4.3.B(1.1)(a) Owner-occupancy (ADU-L), PB-6: Amendments to Zoning Art. 4.3 Accessory Use Regulations and Art. 2 Definitions (ADUs, Affordable Homes Act), Dec. 9, 2024 STM, p. 3
    (a) The owner(s) of the subject premises must live in either the principal single-family dwelling or the ADU-L as his/her/their primary residence except for bona fide temporary absences.
  9. Explanation (PB-6), PB-6: Amendments to Zoning Art. 4.3 Accessory Use Regulations and Art. 2 Definitions (ADUs, Affordable Homes Act), Dec. 9, 2024 STM, p. 4
    the Commonwealth has recently passed the Affordable Homes Act to make ADUs more available, and this article makes the necessary changes to conform to the new state law.
This page summarizes Cohasset's posted bylaws as of 2026-10-03. Bylaws are amended, and individual lots can have overlays, subdivision covenants, or HOA rules that are stricter. Confirm with Building & Inspections Department (Director of Planning, Permits & Inspections) (781-383-4100 ext. 5100 (Building & Inspections); ext. 5103 (Director)) before you build. Spot an error?