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Can I build an ADU in Belmont, Massachusetts?

Belmont, Middlesex County · Bylaws checked 2026-10-03 · Reviewed · 9 cited sections

Allowed

Yes. Belmont's Zoning By-Law allows one accessory dwelling unit by right as an accessory use in the Single Residence and General Residence districts, attached or detached. It can be no larger than half the main home's gross floor area or 900 sq ft, whichever is smaller, and needs one extra parking space unless your lot is within 0.5 miles of a transit station.

Number allowedOne ADU per lot, by right [2]
Zones allowedY (allowed) in Single Residence A-D and General Residence; also Local Business I-III; not in AH, GB, PL [1]
Max sizeSmaller of 1/2 the principal dwelling's gross floor area or 900 sq ft [3]
Parking1 additional space; none on a Transit-Adjacent Lot (within 0.5 mile of a transit station) [6]
Detached ADU spacingAt least 5 ft from other structures; not between the street and the main house (unless in an existing structure) [7]
OwnershipADU can't be sold separately from the main house [4]
Short-term rentalsProhibited in ADUs [5]
Owner-occupancyNo owner-occupancy requirement in Section 6.14 [2]

Where and how many

The use table marks Accessory Dwelling Units as allowed (Y) in the Single Residence A-D and General Residence districts [1]. Section 6.14, adopted at the March 2025 Special Town Meeting, says only one ADU is permitted by right as an accessory use to another dwelling unit, and it doesn't need its own utility meter [2]. The ADU can be an interior conversion (basement or attic), an addition, a conversion of an existing accessory building, or a new detached building; a detached garage may include a first- or second-story ADU [8].

Size and placement

An ADU can't be larger than half the main home's gross floor area or 900 sq ft, whichever is smaller [3]. It follows the height and front setback rules for the main house and the 5-foot side/rear setback for accessory buildings, and (unless it's in an existing structure) can't sit closer to the front lot line than the main house. A new detached ADU must be at least 5 ft from other structures and can't be placed between the road and the main house [7]. New ADU additions or detached units get slightly higher lot-coverage allowances under Table 6.14.5(d) (for example 25% coverage and 45% open space in Single Residence A).

Parking

One extra parking space is required unless the lot is Transit-Adjacent, in which case none is needed [6]. The ADU generally shares the main house's driveway opening.

Ownership and rentals

The ADU can't be sold separately; both units stay in single ownership [4]. Short-term rentals (as defined in M.G.L. c. 64G) are prohibited in ADUs [5]. Section 6.14 has no owner-occupancy requirement.

Approvals

You apply to the Building Division for a building permit and/or certificate of occupancy with a plot plan, a certified plot plan stamped by a Massachusetts land surveyor, and elevation drawings if the exterior changes. Design and Site Plan Review is also required if the ADU exceeds the main house's height, is or adds a second story to an accessory building, pushes lot coverage or open space past the base limits, or involves a nonconforming building [9]. In a local historic district you also need a Certificate of Appropriateness from the Historic District Commission.

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

Next steps

  1. Check your zoning district and whether your lot is within 0.5 mile of a subway, commuter rail or bus station (Transit-Adjacent) with the Office of Planning & Building (617-993-2650).
  2. Confirm the size cap: measure your home's gross floor area; the ADU can be at most half of it or 900 sq ft.
  3. Hire a Massachusetts-registered land surveyor for a certified plot plan showing both units, all buildings, a zoning matrix and parking.
  4. Apply to the Building Division for a building permit; ask whether your project triggers Design and Site Plan Review (second story on an accessory building, exceeding the main house height, extra lot coverage, nonconformities).
  5. If you're in a local historic district, get a Certificate of Appropriateness from the Historic District Commission first.

Common questions

Can I build an ADU in Belmont, Massachusetts?
Yes. Belmont's Zoning By-Law allows one accessory dwelling unit per lot by right in the Single Residence and General Residence districts, attached or detached. You still need a building permit from the Building Division.
How big can an ADU be in Belmont, Massachusetts?
No larger than half the main home's gross floor area or 900 sq ft, whichever is smaller.
Does an ADU need extra parking in Belmont, Massachusetts?
Yes, one additional parking space is required, unless the lot is within 0.5 mile of a transit station (a Transit-Adjacent Lot), in which case none is needed.
Can I rent my ADU on Airbnb in Belmont, Massachusetts?
No. Short-term rentals, as defined in state law, are prohibited in ADUs. Section 6.14 has no owner-occupancy requirement.

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

  1. §3.3 Schedule of Use Regulations, Accessory Uses, Zoning By-Law (as amended through March 3, 2025; approved July 2, 2025) - complete town PDF, p. 39
    Accessory Dwelling Units (See Y Y N Y Y Y N N definitions in §1.4 and §6.14)
  2. §6.14.3 General Requirements, Zoning By-Law (as amended through March 3, 2025; approved July 2, 2025) - complete town PDF, p. 108
    Only one ADU is permitted by right in accordance with § 3.3 as an accessory use to another Dwelling Unit, subject to the following: The ADU shall not be required to be served by any separate utility meter.
  3. §6.14.5(a) Dimensional Requirements, Zoning By-Law (as amended through March 3, 2025; approved July 2, 2025) - complete town PDF, p. 108
    a) A Protected Use ADU may not be larger in Gross Floor Area than 1/2 the Gross Floor Area of the Principal Dwelling or 900 square feet, whichever is smaller.
  4. §6.14.4(a) Prohibitions, Zoning By-Law (as amended through March 3, 2025; approved July 2, 2025) - complete town PDF, p. 108
    a) The ADU may not be sold or title transferred separate and apart from the principal dwelling to which it is an Accessory Use. The Principal Dwelling and the ADU shall remain in common/single ownership and shall not be severed in ownership.
  5. §6.14.4(b) Prohibitions, Zoning By-Law (as amended through March 3, 2025; approved July 2, 2025) - complete town PDF, p. 108
    b) Short-Term Rentals, as defined in Section 6.14.2 above, are prohibited in ADUs.
  6. §6.14.6(a) Parking Requirements, Zoning By-Law (as amended through March 3, 2025; approved July 2, 2025) - complete town PDF, p. 109
    a) One (1) additional Parking Space shall be required for an ADU unless the property is located on a Transit-Adjacent Lot, in which case no additional Parking Space is required.
  7. §6.14.7(a)-(b) Design Requirements, Zoning By-Law (as amended through March 3, 2025; approved July 2, 2025) - complete town PDF, p. 109
    a) A detached ADU shall be no less than 5’ from other Structures on the Lot. b) Unless located within a pre-existing Structure, a detached ADU shall not be located between a roadway and the Principal Dwelling.
  8. §4.3.5(a)(2) Accessory Buildings, Zoning By-Law (as amended through March 3, 2025; approved July 2, 2025) - complete town PDF, p. 53
    2. Notwithstanding the requirements of a)1. Above or b) or c) below, a detached private garage may include a first or second story accessory dwelling unit provided that the accessory building complies with applicable dimensional requirements in Sections 4.2.1 and 4.2.2 herein as well as any additional or superseding dimensional requirements in Section 6.14.
  9. §6.14.8(d) Application, Zoning By-Law (as amended through March 3, 2025; approved July 2, 2025) - complete town PDF, p. 110
    d) Design and Site Plan Review shall be required for any ADU that meets any of the following criteria:
This page summarizes Belmont'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 Office of Planning & Building - Building Division (Inspector of Buildings / Code Enforcement) (617-993-2650 (code enforcement 617-993-2657)) before you build. Spot an error?