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Can I build an ADU in Plymouth, Massachusetts?

Plymouth, Plymouth County · Bylaws checked 2026-10-15 · Reviewed · 7 cited sections

Allowed with a permit

Yes. Plymouth's zoning bylaw (October 2025 version) allows one accessory dwelling unit, attached or detached, per lot by right in all Residential and Mixed Use districts, up to 900 sq ft, with a zoning permit and then a building permit from Inspectional Services. The ADU can't be a short-term rental. The bylaw's definition still says the main house must be owner-occupied, but state law since Feb. 2, 2025 bars owner-occupancy requirements for one ADU per lot.

Where allowedBy right in all Residential and Mixed Use zoning districts (RR, R-40, R-25, R-20SL, R-20MF, DH, LI/WF, WF) [2]
Number per lotOne ADU [1]
Max floor area900 sq ft [3]
Short-term rentalNot allowed in an ADU (MGL c. 64G) [3]
Owner-occupancy (bylaw text)Definition says accessory to an owner-occupied single-family home; state law since Feb. 2, 2025 may override this [1]
ParkingMust meet zoning parking rules based on total bedrooms on the lot [3]
ApprovalsZoning application first, then building permit [7]

Where and what kind

Plymouth's zoning bylaw defines an ADU as a self-contained detached or attached unit with its own sleeping, cooking and sanitary facilities on the same lot as a single-family home, and allows only one per lot [1]. Section 203-17, added at the April 2024 Town Meeting, makes an ADU allowed by right in all Residential and Mixed Use districts [2], and the use table marks ADUs as permitted by right (Y) in the five residential districts (RR, R-40, R-25, R-20SL, R-20MF) and the three mixed-use districts (DH, LI/WF, WF), and not permitted (X) in the commercial districts [5]. If the house or lot is nonconforming, an ADU is still allowed as long as it creates no new zoning nonconformity and meets health and building codes [2]. Condominiums and townhouses don't count as single-family dwellings for this purpose [4].

Size, use and parking

The ADU must stay in single ownership with the main house, have a separate entrance that meets building-code egress rules, be no larger than 900 sq ft, and meet the bylaw's parking requirements based on the total bedroom count on the lot [3]. It may not be used as a short-term rental as defined in MGL c. 64G [3]. The bylaw doesn't set a separate ADU parking number.

Owner-occupancy and state law

The definition still describes an ADU as accessory to an owner-occupied single-family dwelling [1], and the town's ADU page repeats that wording [6]. Massachusetts' ADU law (in effect since Feb. 2, 2025) bars towns from requiring owner-occupancy for one protected ADU, so state law may override that provision. State law protects an ADU of up to 900 sq ft or half the main home's floor area, whichever is smaller; Plymouth's flat 900 sq ft local cap can be more generous for smaller homes, so check how the town applies it. State law also bars requiring more than one extra parking space for a protected ADU (none near transit), so state law may override the bylaw's bedroom-based parking rule.

Approval process

The town's ADU page says to file a zoning application first, with a site plan showing setbacks and dimensions and a floor plan of existing and proposed living space, then a building permit (new construction for a detached unit, addition/alteration for an attached one). Building permit applications are accepted only after the zoning permit is approved [7].

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

Next steps

  1. Confirm your zoning district on the Official Zoning Map and that the lot will have only one ADU.
  2. File a zoning application with Inspectional Services (26 Court Street, 508-747-1620) with a site plan showing setbacks and dimensions and a floor plan of existing and proposed living space.
  3. After the zoning permit is approved, file a building permit: new construction for a detached ADU, or addition/alteration for one inside or attached to the house.
  4. If you're on septic, check Title 5 capacity with the Board of Health before adding bedrooms.

Common questions

Can I build an ADU in Plymouth, MA?
Yes. Plymouth's zoning bylaw allows one accessory dwelling unit, attached or detached, per lot by right in all Residential and Mixed Use districts, with a zoning permit and then a building permit from Inspectional Services. ADUs aren't permitted in the commercial districts.
How big can an ADU be in Plymouth, Massachusetts?
Up to 900 sq ft, with one ADU per lot. The ADU needs a separate entrance, must stay in single ownership with the main house, and must meet the bylaw's parking rules based on the total bedrooms on the lot.
Does the owner have to live in the house for an ADU in Plymouth, MA?
It's unsettled. Plymouth's bylaw definition still says the ADU is accessory to an owner-occupied single-family home, but state law since Feb. 2, 2025 bars towns from requiring owner-occupancy for one protected ADU, so state law may override it.
Can I use my ADU as a short-term rental in Plymouth, Massachusetts?
No. Plymouth's zoning bylaw says an ADU may not be used as a short-term rental as defined in MGL c. 64G.

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 Plymouth's published bylaws. Section numbers are as printed in the source.

  1. §201-3 Definitions, Accessory Dwelling Unit (ADU), Zoning Bylaw of the Town of Plymouth (as amended through October 2025), p. 5
    ACCESSORY DWELLING UNIT (ADU) – A self-contained detached or attached housing unit, inclusive of sleeping, cooking and sanitary facilities on the same lot as the principal single-family dwelling and that is accessory to an owner-occupied Single- Family Dwelling located in a Residential or Mixed Use Zoning District. Only one ADU shall be allowed on a Lot.
  2. §203-17 Accessory Dwelling Units, Zoning Bylaw of the Town of Plymouth (as amended through October 2025), p. 76
    An Accessory Dwelling Unit is allowed by right in all Residential and Mixed Use zoning districts provided that: 1. The principal Single-Family Dwelling and the Lot on which it is located conforms to the area and dimensional requirements of the underlying Zoning District; or
  3. §201-3 Definitions, Accessory Dwelling Unit (ADU), Zoning Bylaw of the Town of Plymouth (as amended through October 2025), p. 5
    2. Not be used as a short-term rental unit as defined by MGL c. 64G; 3. Be located on the same Lot as the principal Single-Family Dwelling; 4. Maintain a separate entrance, either directly from the outside or through an entry hall or corridor shared with the principal dwelling sufficient to meet the requirements of the state building code for safe egress; 5. Not be larger in floor area than 900 square feet; and 6. Complies with the parking requirements (based on total bedroom count on the Lot) of the Zoning Bylaw.
  4. §203-17 Accessory Dwelling Units, Zoning Bylaw of the Town of Plymouth (as amended through October 2025), p. 76
    For the purposes of permitting Accessory Dwelling Units within the Town under this Section of the Bylaw, the following shall not be considered single-family dwellings: 1. A condominium as otherwise defined by Massachusetts General Laws; or 2. A townhouse dwelling as defined with Section 201-3 of this Bylaw.
  5. Use Table, F. Accessory, line 5, Zoning Bylaw of the Town of Plymouth (as amended through October 2025), p. 193
    5 Accessory Dwelling Units Y Y Y Y Y Y Y
  6. ADU Information page (quoting §201-3), Accessory Dwelling Unit (ADU) Information - Inspectional Services
    A self-contained detached or attached housing unit, inclusive of sleeping, cooking and sanitary facilities on the same lot as the principal single-family dwelling and that is accessory to an owner-occupied Single-Family Dwelling located in a Residential or Mixed Use Zoning District.
  7. ADU Information page, How to Apply, Accessory Dwelling Unit (ADU) Information - Inspectional Services
    *Please note that building permit applications for ADU's will only be accepted if you have an approved zoning permit
This page summarizes Plymouth's posted bylaws as of 2026-10-15. Bylaws are amended, and individual lots can have overlays, subdivision covenants, or HOA rules that are stricter. Confirm with Department of Inspectional Services (Building Commissioner / zoning enforcement), 26 Court Street (508-747-1620) before you build. Spot an error?