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Can I build an ADU in Canton, Massachusetts?

Canton, Norfolk County · Bylaws checked 2026-10-20 · Reviewed · 14 cited sections

Allowed with a permit

Yes. Canton allows one accessory dwelling unit (attached or detached) by right on any lot in a district that allows single-family homes, with a building permit and certificate of occupancy from the Building Commissioner. It can be no larger than half the main home's floor area or 900 sq ft, whichever is smaller, and needs one extra parking space unless you are within half a mile of a transit station.

ADUs allowed by right1 per lot, in all districts allowing single-family homes (by right or special permit) [1]
Maximum sizeSmaller of 900 sq ft or 1/2 the main home's gross floor area [2]
ApprovalBuilding permit and certificate of occupancy (Building Commissioner) [3]
Additional ADUsSpecial permit from the Board of Appeals [4]
Parking1 extra off-street space; none within 0.5 mile of a transit station [6]
Owner-occupancyNot required (no such clause in Section 8.1) [1]
Minimum lot sizeNone (the 5,000 sq ft minimum was disapproved by the Attorney General in 2025) [10]

Current rule (Section 8.1 as rewritten in 2026)

Canton's ADU bylaw is Zoning By-Law Section 8.1, rewritten by Article 43 of the May 11, 2026 Annual Town Meeting and approved by the Attorney General on September 22, 2026. One ADU is allowed as of right in every district where single-family homes are allowed by right or by special permit [1]. The ADU definition (Article 44) covers a unit either inside the main home or elsewhere on the same lot, so attached, internal and detached units all qualify [8]. The posted "Zoning By-Laws 2026" PDF predates this vote and still prints the older text, including a 5,000 sq ft minimum lot and a "same setbacks as the primary residence" clause. The Attorney General disapproved both in 2025 [10][14], so they are void, and the 2026 rewrite drops them from the text.

Size, entrance and design

The ADU can be no larger than half the main home's gross floor area or 900 sq ft, whichever is smaller [2]. It needs its own entrance, either from outside or through a shared hall that meets building-code egress rules. It must look like part of the main structure: stairs to upper floors go inside the exterior walls, and new entrances go on the side or rear [5][13]. ADUs must meet the ordinary dimensional rules in Section 4.0 [12]. The bylaw doesn't say whether a detached ADU follows the house setbacks or the smaller accessory-structure setbacks; the Attorney General's 2025 decision says the town can't apply a stricter setback than the more permissive of those [14]. Confirm the setback for a detached ADU with the Building Commissioner.

Parking

You need one off-street space for the ADU on top of what the main home requires. No extra space is required within 0.5 mile of a commuter rail, subway, ferry or bus station [6][7]. One driveway serves both units, though a detached ADU may have its own parking area.

Permits and more than one ADU

The Building Commissioner issues the building permit, and the unit can't be lived in until a certificate of occupancy is issued [3]. A second or further ADU on the same lot needs a Board of Appeals special permit [4]. Section 8.1 has no owner-occupancy requirement.

Stale use-table row

The Table of Use Regulations still lists "Accessory Apartment in single family dwelling pursuant to Section 8.1" as BA (Board of Appeals special permit) [9]. That row is left over from the old accessory-apartment bylaw. "Accessory Apartment" now means only units created by special permit before August 6, 2024 [11]. Under state law, a special-permit requirement could not be applied to a first ADU anyway.

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

Next steps

  1. Measure your home's gross floor area to confirm your size cap (half of it, up to 900 sq ft).
  2. Have a certified plot plan and floor plans prepared showing the ADU, its separate entrance and the parking space (unless you are within 0.5 mile of a transit station).
  3. Apply for a building permit through Canton Online Permitting with the Building Department, Memorial Hall, 801 Washington St. (781-821-5003).
  4. If you are on septic, get Board of Health sign-off for the added bedrooms. If the site is within 100 ft of a wetland, check with the Conservation Commission first.
  5. Call for inspections and get the certificate of occupancy before anyone moves in.

Common questions

Are ADUs allowed in Canton, Massachusetts?
Yes. Canton allows one accessory dwelling unit, attached or detached, by right on a lot in any district that allows single-family homes, with a building permit and certificate of occupancy from the Building Commissioner. Section 8.1 has no owner-occupancy requirement.
How big can an ADU be in Canton, MA?
No larger than half the main home's gross floor area or 900 sq ft, whichever is smaller. Canton has no minimum lot size for an ADU: the Attorney General disapproved the old 5,000 sq ft minimum in 2025, so it is void even though the posted zoning PDF still prints it.
What are the setbacks for a detached ADU in Canton, Massachusetts?
It depends, and it isn't settled. ADUs must meet the dimensional rules in Section 4.0, but the bylaw doesn't say whether a detached ADU follows the house setbacks or the smaller accessory-structure setbacks. The Attorney General's 2025 decision says the town can't apply a stricter setback than the more permissive of those. Confirm with the Building Commissioner.
Does an ADU need extra parking in Canton, MA?
Usually one off-street space on top of what the main home requires, but none if the ADU is within 0.5 mile of a commuter rail, subway, ferry or bus station. One driveway serves both units, though a detached ADU may have its own parking area.
Can I build a second ADU in Canton, Massachusetts?
Only with a special permit. The first ADU is allowed by right, but a second or further ADU on the same lot needs a special permit from the Board of Appeals.

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

  1. Zoning By-Law §8.1.2 (as amended by 2026 ATM Art. 43), Attorney General approval (Sept 22, 2026) of 2026 Annual Town Meeting Articles 24-33 and 43-47, with certified votes and text, p. 25
    One Accessory Dwelling Unit (ADU) shall be allowed as-of-right in all zoning districts in which single-family homes are allowed as-of-right or by Special Permit, subject to the following restrictions:
  2. Zoning By-Law §8.1.2 (as amended by 2026 ATM Art. 43), Attorney General approval (Sept 22, 2026) of 2026 Annual Town Meeting Articles 24-33 and 43-47, with certified votes and text, p. 25
    The ADU is not larger in gross floor area than one half (1/2) the gross floor area of the principal dwelling or 900 square feet, whichever is smaller;
  3. Zoning By-Law §8.1.2 (as amended by 2026 ATM Art. 43), Attorney General approval (Sept 22, 2026) of 2026 Annual Town Meeting Articles 24-33 and 43-47, with certified votes and text, p. 25
    No ADU shall be constructed without issuance of a building permit by the Building Commissioner; and
  4. Zoning By-Law §8.1.3 (as amended by 2026 ATM Art. 43), Attorney General approval (Sept 22, 2026) of 2026 Annual Town Meeting Articles 24-33 and 43-47, with certified votes and text, p. 25
    The Board of Appeals may issue a Special Permit for additional ADUs over and above the first ADU allowed as-of-right on a lot.
  5. Zoning By-Law §8.1.4(A) (as amended by 2026 ATM Art. 43), Attorney General approval (Sept 22, 2026) of 2026 Annual Town Meeting Articles 24-33 and 43-47, with certified votes and text, p. 25
    All stairways to second or third stories shall be enclosed within the exterior walls of the dwelling;
  6. Zoning By-Law §8.1.6(A) (as amended by 2026 ATM Art. 43), Attorney General approval (Sept 22, 2026) of 2026 Annual Town Meeting Articles 24-33 and 43-47, with certified votes and text, p. 25
    There shall be one (1) off-street parking space per ADU in addition to the parking spaces that are required by the underlying zoning district for the Principle Use;
  7. Zoning By-Law §8.1.6(B) (as amended by 2026 ATM Art. 43), Attorney General approval (Sept 22, 2026) of 2026 Annual Town Meeting Articles 24-33 and 43-47, with certified votes and text, p. 25
    No additional parking spaces shall be required if the ADU is located within 0.5 miles from a commuter rail station, subway station, ferry terminal, or bus station;
  8. Zoning By-Law §11.0 Definitions (as amended by 2026 ATM Art. 44), Attorney General approval (Sept 22, 2026) of 2026 Annual Town Meeting Articles 24-33 and 43-47, with certified votes and text, p. 27
    Accessory Dwelling Unit: A secondary dwelling unit, but not a mobile home, located either within or on the same lot as a principle dwelling.
  9. Attachment A, Table of Use Regulations, J. Accessory Uses, Zoning By-Laws 2026 (amended through STM 2025), p. 23
    Accessory Apartment in single family BA BA BA BA N N N N N dwelling pursuant to Section 8.1
  10. AG decision, Case #11654, §VI.B.2, Attorney General partial approval (June 4, 2025) of Nov 18, 2024 STM Articles 6 and 8 (ADU bylaw and definitions), p. 10
    Because the Regulations, 760 CMR 71.03 (3)(b)(2), prohibit a Town from requiring a minimum lot size for a Protected Use ADU, Section 8.1.2 (B) conflicts with the Regulations and we therefore disapprove the text shown above in bold and underline.
  11. §11.0 Definitions, Accessory Apartment, Zoning By-Laws 2026 (amended through STM 2025), p. 175
    Accessory Apartment: A pre-existing non-conforming dwelling unit created in a single- family dwelling by special permit granted by the zoning board of appeals and recorded at the Norfolk County registry of deeds prior to the adoption of An Act Relative to the Affordable Homes Act by the 193rd Massachusetts General court on August 6, 2024.
  12. Zoning By-Law §8.1.5 (as amended by 2026 ATM Art. 43), Attorney General approval (Sept 22, 2026) of 2026 Annual Town Meeting Articles 24-33 and 43-47, with certified votes and text, p. 25
    Accessory Dwelling Units shall be subject to all applicable dimensional requirements in Section 4.0 of the Town of Canton Zoning By-Laws.
  13. Zoning By-Law §8.1.4(B) (as amended by 2026 ATM Art. 43), Attorney General approval (Sept 22, 2026) of 2026 Annual Town Meeting Articles 24-33 and 43-47, with certified votes and text, p. 25
    Any new, additional entrance to an existing dwelling shall be located on the side or in the rear of the dwelling; and
  14. AG decision, Case #11654, §VI.B.2, Attorney General partial approval (June 4, 2025) of Nov 18, 2024 STM Articles 6 and 8 (ADU bylaw and definitions), p. 10
    Second, we disapprove and delete the portion of Section 8.1.2 (D), as shown above in bold and underline, that requires the ADU to have the same dimensional setbacks as “the primary residence” because this requirement conflicts with 760 CMR 71.03 (3), as quoted above, that prohibits the Town from imposing a dimensional requirement on an ADU that is more restrictive than that required for the Principal Dwelling, or a Single-family Residential Dwelling or an accessory structure in the Zoning District in which the Protected Use ADU is located and requires the more permissive dimensional requirement to be applied.
This page summarizes Canton's posted bylaws as of 2026-10-20. Bylaws are amended, and individual lots can have overlays, subdivision covenants, or HOA rules that are stricter. Confirm with Building Department (Building Commissioner Edward Walsh; Zoning Enforcement Officer in the same office), Lower Level, Memorial Hall, 801 Washington Street, Canton MA 02021 (781-821-5003) before you build. Spot an error?