Town Permit Guide

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Can I build an ADU in Sudbury, Massachusetts?

Sudbury, Middlesex County · Bylaws checked 2026-10-03 · Reviewed · 13 cited sections

Allowed with a permit

Yes. Sudbury's 2025 Section 5500 (adopted at the May 5, 2025 Town Meeting and partly approved by the Attorney General) allows one ADU per lot as of right in Single Residence Districts A and C with a building permit. It can be attached or detached, must be no larger than 900 sq ft or 50% of the main house's floor area (whichever is smaller), needs one off-street parking space (none near transit), and can't be used as a short-term rental.

ApprovalAs of right (no special permit); building permit required [11]
DistrictsSingle Residence A and C, Wayside Inn Historic Preservation Zone, Limited Business, Village Business [1]
Max size900 sq ft or 50% of the main home's gross floor area, whichever is smaller [2]
Detached ADUAllowed; must meet district setbacks (the AG struck the minimum-lot-area condition and the 'principal residence' setback wording) [12]
Number per lot1 [4]
Parking1 dedicated off-street space (none within 0.5 mile of transit); AG struck 'at least' [13]
Short-term rentalProhibited in any ADU [6]
Board of HealthWritten BOH report on sewage/septic required with building permit filing [7]

Allowed as of right

At the May 5, 2025 Annual Town Meeting (Article 28), Sudbury replaced its old special-permit ADU rules with a new Section 5500 to match the state Affordable Homes Act. The Attorney General approved it on October 14, 2025, except for a few provisions it struck as conflicting with state law [11]. One ADU per lot with a principal dwelling may be built as of right in Single Residence Districts A and C, the Wayside Inn Historic Preservation Zone, and the Limited and Village Business Districts [1]. Only one ADU is allowed per lot [4]. A building permit (or Building Department inspection) is required [8].

Size and design

An ADU may be no more than 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is smaller [2]. An attached ADU must keep the look of a dwelling [9] (the AG deleted the bylaw's "single-family" wording), and a detached ADU must look like an accessory structure that matches the main house.

Detached ADUs

As adopted, Section 5522 allowed a detached ADU only on a lot meeting the district's minimum lot area and with the same setbacks as the house [3]. The AG deleted the lot-area condition and the "principal residence" setback wording [12], so a detached ADU must meet the district's setback rules. Under state regulations the town must apply the most permissive of the house or accessory-structure standards; confirm which setback applies with the Building Department.

Parking and rentals

Provide one dedicated off-street space, or none within 0.5 mile of a commuter rail, subway, ferry or bus station [5]. The AG struck the words "at least" because towns can't require more than one space [13]. Garage spaces don't count, and no new curb cut is allowed for an ADU. ADUs may not be used as short-term rentals [6], which state law lets towns prohibit. There is no owner-occupancy requirement, and the ADU can't be sold separately from the house [10].

Septic and approvals

You must give the Building Inspector a written Board of Health report confirming adequate sewage and drainage provisions, including a reserve area [7]. ADUs approved by special permit before February 2, 2025 become protected ADUs if they meet the new rules.

About our sources

Rules come from the Article 28 text as voted, read together with the AG's decision. The codified version on eCode360 couldn't be checked. The 2023 zoning PDF still shows the old special-permit rules (850 sq ft, owner-occupancy), which no longer apply.

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

Next steps

  1. Confirm your lot's zoning district and whether you're within 0.5 mile of a transit station (Building Department, 978-440-5461).
  2. Get a written Board of Health report confirming septic/sewage capacity and reserve area for the ADU.
  3. Apply for a building permit with floor plans showing ADU and main-house gross floor area, egress, fire separation and the parking space.
  4. If your house or lot is nonconforming, ask whether a Section 2400 finding from the Zoning Board of Appeals is needed.

Common questions

Can I build an ADU in Sudbury, Massachusetts?
Yes. Section 5500 allows one ADU per lot as of right in Single Residence Districts A and C with a building permit. It can be attached or detached.
How big can an ADU be in Sudbury, Massachusetts?
No larger than 900 sq ft or 50% of the main house's floor area, whichever is smaller.
Does an ADU need parking in Sudbury, Massachusetts?
Yes, 1 dedicated off-street space, with none required within 0.5 mile of transit.
Can I use my ADU as an Airbnb in Sudbury, Massachusetts?
No. Short-term rental is prohibited in any ADU. A written Board of Health report on sewage or septic is also required with the building permit filing.

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

  1. Sec. 5520 Conditions and Requirements, ADU zoning amendment - new Section 5500 and 7110 definition (2025 Annual Town Meeting article), p. 1
    Notwithstanding anything to the contrary in this Zoning Bylaw, one Accessory Dwelling Unit (ADU) per lot containing a principal dwelling may be constructed as of right in the Single Residence District "A", "C", Wayside Inn Historic Preservation Zone, Limited Business District, or Village Business District subject to the following:
  2. Sec. 5521, ADU zoning amendment - new Section 5500 and 7110 definition (2025 Annual Town Meeting article), p. 1
    5521. The Accessory Dwelling Unit shall contain no more than 900 square feet, or shall occupy no more than 50% of the gross floor area of the principal dwelling, whichever is smaller.
  3. Sec. 5522, ADU zoning amendment - new Section 5500 and 7110 definition (2025 Annual Town Meeting article), p. 1
    5522. An Accessory Dwelling Unit may be built in a detached accessory structure separate from the main dwelling provided: (1) the lot meets the current minimum zoning requirement for lot area in the district in which the lot is located, (2) the detached structure meets the same minimum setback requirements that apply to the
  4. Sec. 5523, ADU zoning amendment - new Section 5500 and 7110 definition (2025 Annual Town Meeting article), p. 2
    5523. There will be no more than one Accessory Dwelling Unit per building lot.
  5. Sec. 5526 Off-street Parking, ADU zoning amendment - new Section 5500 and 7110 definition (2025 Annual Town Meeting article), p. 2
    5526. Off-street Parking. There shall be at least one dedicated off-street parking for the Accessory Dwelling Unit or none if you are within 0.5 miles from a commuter rail station, subway station, ferry terminal, or bus station.
  6. Sec. 5527, ADU zoning amendment - new Section 5500 and 7110 definition (2025 Annual Town Meeting article), p. 3
    5527. It is prohibited for any ADU in the Town of Sudbury to be used as a Short- Term Rental as defined in Section 1 of Chapter 64G of the General Laws.
  7. Sec. 5531 Reports, ADU zoning amendment - new Section 5500 and 7110 definition (2025 Annual Town Meeting article), p. 3
    5531. In order to ensure compliance, the applicant shall obtain and submit to the Building Inspector as part of the filing for a Building Permit, a written report of the Board of Health certifying that the conditions of subsection 5524 have been met.
  8. Sec. 5541 Building Permit Required, ADU zoning amendment - new Section 5500 and 7110 definition (2025 Annual Town Meeting article), p. 3
    5541. Building Permit Required. A Building Permit, or inspection by the Building Department, shall be required to create an Accessory Dwelling Unit.
  9. Sec. 5525A, ADU zoning amendment - new Section 5500 and 7110 definition (2025 Annual Town Meeting article), p. 2
    5525A. An attached Accessory Dwelling Unit will be designed so that the appearance of the structure remains that of a single-family dwelling and its associated accessory structures
  10. Sec. 5542 No Separate Conveyance, ADU zoning amendment - new Section 5500 and 7110 definition (2025 Annual Town Meeting article), p. 3
    The Accessory Dwelling Unit shall remain accessory to a principal dwelling and the ownership of the Accessory Dwelling Unit shall not be conveyed or otherwise transferred separately from the principal dwelling.
  11. AG decision, Case #11774, p. 1, Attorney General decision on 2025 ATM Article 28 (ADU zoning), Case #11774, October 14, 2025, p. 1
    Under Article 28 the Town amended its zoning by-laws to allow Accessory Dwelling Units (“ADUs”) as of right in compliance with G.L. c. 40A, § 3 and the implementing Regulations promulgated by the Executive Office of Housing and Livable Communities (“EOHLC”), 760 CMR 71.00, “Protected Use Accessory Dwelling Units” (“Regulations”).
  12. AG decision, Sec. IV.B (Sec. 5522), Attorney General decision on 2025 ATM Article 28 (ADU zoning), Case #11774, October 14, 2025, p. 6
    We disapprove the text shown above in bold and underline that imposes a minimum lot size requirement on an ADU and requires the ADU to meet the dimensional standards of the principal residence, as explained below.
  13. AG decision, Sec. IV.C (Sec. 5526), Attorney General decision on 2025 ATM Article 28 (ADU zoning), Case #11774, October 14, 2025, p. 7
    We disapprove the parking provision shown above in bold and underline, that requires “at least” one off-street parking space, because it conflicts with G.L. c. 40A, § 3 and the Regulations.
This page summarizes Sudbury'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 Department (Building Inspector / Zoning Enforcement Agent) ((978) 440-5461) before you build. Spot an error?