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Can I build an ADU in Acton, Massachusetts?

Acton, Middlesex County · Bylaws checked 2026-10-19 · Reviewed · 12 cited sections

Allowed with a permit

Yes. Acton allows one accessory dwelling unit (ADU) per lot as of right in single-family residential districts. It must be smaller than half the main home's gross floor area or 900 sq ft, whichever is smaller, and you need a building permit and an occupancy permit from the Building Commissioner.

How allowedAs of right in single-family residential districts (no special permit) [1]
Number per lot1 ADU [2]
Max sizeLess than 50% of the main home's gross floor area or 900 sq ft, whichever is smaller [3]
Basement ADUs in use before May 6, 2025No maximum floor area [4]
Detached building existing before April 2, 2019 (not enlarged)ADU up to 2,000 sq ft (excluding attic/basement) [5]
Parking1 extra space if the lot is outside a 0.5-mile radius of a transit station [6]
Detached ADU foundationMust be built on a foundation [7]
PermitsBuilding and Occupancy Permits from the Building Commissioner [8]

As of right under state law

Acton's Zoning Bylaw §3.8.1.6 says its purpose is to let ADUs be built as of right in single-family residential districts under M.G.L. c. 40A, §3 (as amended by Chapter 150 of the Acts of 2024) and 760 CMR 71.00 [1]. No special permit or owner-occupancy rule appears in §3.8.1.6. Only one ADU is allowed per lot [2].

Attached or detached, and how big

  • Standard limit: the ADU's gross floor area must be less than 50% of the main home's, or 900 sq ft, whichever is smaller [3].
  • Existing basement units: a basement ADU used continuously since before May 6, 2025 has no maximum size [4].
  • Older detached buildings: a detached building (such as a barn or garage) that has existed since before April 2, 2019 and hasn't been enlarged can hold an ADU of up to 2,000 sq ft, not counting attic or basement [5].
  • A detached ADU must be built on a foundation [7].
  • §3.3(b) lets an ADU in a detached building be a second dwelling building on the lot without a special permit [12].

Parking

If your lot is outside a 0.5-mile radius of a transit station, the ADU needs one additional parking space for its resident [6]. Inside that radius, no extra space is required. The general two-spaces-per-unit rule doesn't apply to ADUs [10].

Dividing the lot later

You can't split off a detached ADU onto its own lot unless both resulting lots and buildings meet all the district's area, frontage, width and yard rules [11].

Permits

No ADU may be built or occupied without Building and Occupancy Permits from the Building Commissioner [8]. Normal zoning setbacks and height limits still apply, and septic (Title 5) and wetland rules may also affect where it goes.

Note: §3.3(c) still lists an ADU under §3.8.1.6 among uses that need a special permit to be a second dwelling building on a lot [9], even though §3.3(b) exempts detached ADUs [12]. Since February 2, 2025, state law bars requiring a special permit for one protected ADU per lot, so state law may override that leftover wording.

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

Next steps

  1. Confirm your lot is in a single-family residential district and measure the main home's gross floor area to work out your size limit (under 50% or 900 sq ft, whichever is smaller).
  2. Check whether your lot is within 0.5 mile of a transit station; if not, plan one extra parking space for the ADU.
  3. Apply for a building permit online with the Acton Building Department (978-929-6633) with plans and a plot plan showing setbacks; check septic and wetland setbacks early.
  4. Get an occupancy permit from the Building Commissioner before anyone moves in.

Common questions

Can I build an ADU in Acton, Massachusetts?
Yes. Acton's Zoning Bylaw allows one accessory dwelling unit per lot as of right in single-family residential districts, with no special permit or owner-occupancy rule in §3.8.1.6. You still need building and occupancy permits from the Building Commissioner.
How big can an ADU be in Acton, MA?
The ADU must be smaller than half the main home's gross floor area or 900 sq ft, whichever is smaller. A basement ADU in continuous use since before May 6, 2025 has no maximum, and a detached building that has existed since before April 2, 2019 and hasn't been enlarged can hold an ADU of up to 2,000 sq ft.
Does an ADU need extra parking in Acton, Massachusetts?
It depends on location. If your lot is outside a 0.5-mile radius of a transit station, the ADU needs one additional parking space for its resident; inside that radius, no extra space is required.
Does Acton, MA require a special permit for a detached ADU?
No special permit appears in the ADU section, and §3.3(b) lets a detached ADU be a second dwelling building on a lot. §3.3(c) still lists ADUs among special-permit uses, but since February 2, 2025 state law bars requiring a special permit for one protected ADU per lot, so state law may override that leftover wording.
Can I build a detached ADU on my property in Acton, Massachusetts?
Yes. A detached ADU is allowed but must be built on a foundation, and normal setbacks and height limits still apply. You can't later split it onto its own lot unless both resulting lots and buildings meet all the district's area, frontage, width and yard rules.

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

  1. §3.8.1.6 Accessory Dwelling Units, Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 29
    The purpose of this Section is to allow for Accessory DWELLING UNITS (ADUs), as defined under M.G.L. c. 40A, §1A, to be built as-of-right in Single-Family Residential Zoning Districts in accordance with M.G.L. c. 40A, § 3, as amended by Section 8 of Chapter 150 of the Acts of 2024, and the regulations promulgated at 760 CMR 71.00.
  2. §3.8.1.6(a), Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 29
    a) There shall be not more than one accessory DWELLING UNIT on a LOT.
  3. §3.8.1.6(b), Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 29
    b) The GROSS FLOOR AREA of an accessory DWELLING UNIT shall be less than fifty percent of the GROSS FLOOR AREA of the Principal Unit or 900 square feet, whichever is smaller.
  4. §3.8.1.6(c), Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 29
    c) An Accessory DWELLING UNIT located in a basement area of a PRINCIPAL DWELLING UNIT and that has been continuously used since before May 6, 2025 shall have no maximum GROSS FLOOR AREA.
  5. §3.8.1.6(d), Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 29
    d) If a detached BUILDING on a LOT has existed since before April 2, 2019 and its foot print and floor area, including the area of interior garage or parking spaces, is not expanded or enlarged thereafter, an Accessory DWELLING UNIT in such a detached BUILDING may have a GROSS FLOOR AREA of up to 2000 square feet, not including attic or basement areas.
  6. §3.8.1.6(e), Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 29
    e) An Accessory DWELLING UNIT located on a lot outside a 0.5-mile radius of a transit station shall have one additional parking space dedicated for use by the resident of the Accessory DWELLING UNIT.
  7. §3.8.1.6(g), Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 29
    g) Any Accessory DWELLING UNIT in a detached BUILDING shall be constructed on a foundation.
  8. §3.8.1.6(h), Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 29
    h) No Accessory DWELLING UNIT permitted under this Section shall be constructed and occupied without Building and Occupancy Permits issued by the Building Commissioner.
  9. §3.3 Residential Uses, Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 18
    c) where a special permit has been granted for the following: an Independent SENIOR Residence under Section 9B of this Bylaw; an Accessory DWELLING UNIT as provided under Section 3.8.1.6; a golf course under Section 3.5.17 of this bylaw.
  10. §6.3.1.1 Parking – Dwelling, Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 118
    6.3.1.1 Dwelling Two spaces for each DWELLING UNIT, except for an Accessory DWELLING UNIT as defined in Section 3.8.1.6.
  11. §3.8.1.6(f), Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 29
    f) A LOT containing a BUILDING with a Principal Unit and an Accessory DWELLING UNIT within a detached BUILDING shall not be further divided resulting in the separation of the Principal Unit and the Accessory DWELLING UNIT, unless both resulting LOTS and the BUILDINGS thereon meet all minimum area, FRONTAGE, width and yard requirements of the applicable zoning district.
  12. §3.3(b) Residential Uses, Zoning Bylaw (revised up to and including the May 2026 Annual Town Meeting), p. 18
    b) for the following USES: Nursing Home; Full Service Retirement Community; Assisted Living Residence as defined in this Bylaw or in MGL Ch. 19D; and Accessory DWELLING UNIT in a detached BUILDING as provided under Section 3.8.1.6;
This page summarizes Acton's posted bylaws as of 2026-10-19. Bylaws are amended, and individual lots can have overlays, subdivision covenants, or HOA rules that are stricter. Confirm with Building Department (Building Commissioner / Zoning Enforcement Officer), Town Hall, 472 Main Street, Acton MA 01720 (978-929-6633) before you build. Spot an error?