Town Permit Guide

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Can I build an ADU in Chatham, Massachusetts?

Chatham, Barnstable County · Bylaws checked 2026-10-03 · Reviewed · 9 cited sections

Allowed with a permit

Yes. Chatham's zoning bylaw allows one ADU per lot (up to 900 sq ft and 2 bedrooms), by right with a building permit if it's inside or attached to the house. The bylaw requires a ZBA special permit for a detached ADU, caps permits at 10 a year and requires a 12-month lease; state law likely overrides the special permit and the caps.

Attached or internal ADUBy right (building permit), max 10 per year [1]
Detached ADU (bylaw text)Special permit from the Zoning Board of Appeals, max 10 per year [2]
Max size900 sq ft of habitable space, 2 bedrooms; can't be enlarged later [3]
ParkingAt least 1 extra off-street space [4]
Rental ruleADU or main house leased 12+ months; no monthly, weekly, daily or vacation rentals [5]
Number per lot1 [6]

Where and how ADUs are allowed

Chatham's Protective Zoning Bylaw (Sec. VII.B.19) lists ADUs as a permitted use in every residential district. An ADU built inside a single-family home or attached to it is allowed by right with a building permit from the Building Commissioner. The bylaw caps these at 10 permits a calendar year [1]. A detached ADU, or one that can't meet the dimensional schedule, needs a special permit from the Zoning Board of Appeals, also capped at 10 a year [2]. You must submit site plans, floor plans and elevations before a permit issues.

Size, parking and septic

  • One ADU per lot [6], with a kitchen and bath.
  • No more than 2 bedrooms and 900 sq ft of habitable space, and it can't be enlarged later [3].
  • At least one extra off-street parking space beyond what the main house needs [4].
  • The Board of Health must confirm the septic system (Title 5) can handle the combined bedrooms. The ADU and house share septic and water service.
  • The ADU can't be sold separately from the main house [7].

Rental and occupancy rules

Either the ADU or the main house must be leased for at least 12 consecutive months with no subletting. Neither unit may be rented monthly, weekly, daily, seasonally or through vacation-rental websites [5]. The ADU must become the renter's principal residence within 30 days [8]. Each year by January 31 the owner must file the lease with the Building Commissioner [9]. If you can't find a tenant, a notarized affidavit lets the ADU sit empty for up to one year.

State law may override parts of this

Since February 2, 2025, state law allows one ADU (up to 900 sq ft or half the main home's floor area, whichever is smaller) by right on any single-family lot. It bars special permits and caps on the number of ADUs. The bylaw's special permit for detached ADUs and its 10-permit yearly caps may therefore not be enforceable. The 12-month lease requirement may also count as an unreasonable restriction, though towns may still ban short-term rentals of ADUs. The bylaw text we reviewed is current through the May 2025 Town Meeting, so ask the Building Division how it applies these rules today.

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

Next steps

  1. Call the Chatham Building Division (508-945-5168) to confirm how the town now handles detached ADUs and the annual permit caps under state law.
  2. Get Board of Health sign-off that your septic system (Title 5) can serve the combined bedroom count.
  3. Prepare site plans, floor plans and elevations showing the ADU and at least one extra off-street parking space, and apply for a building permit.
  4. If the lot is near wetlands or the shore, check with the Conservation Commission before you design.

Common questions

Can I build an ADU in Chatham, Massachusetts?
Yes. Chatham's bylaw allows one ADU per lot, by right with a building permit if it's inside or attached to the house. A detached ADU needs a Zoning Board of Appeals special permit under the bylaw, though state law likely overrides that and the yearly caps.
How big can an ADU be in Chatham, Massachusetts?
Up to 900 sq ft of habitable space and 2 bedrooms, and it can't be enlarged later. At least 1 extra off-street parking space is required.
Can I rent my ADU short-term in Chatham, Massachusetts?
No. The bylaw requires either the ADU or the main house to be leased for at least 12 consecutive months and bars monthly, weekly, daily or vacation rentals. State law may treat the 12-month requirement as an unreasonable restriction, though towns may still ban short-term ADU rentals.
Does Chatham, Massachusetts limit how many ADUs get permitted each year?
The bylaw caps attached and detached ADU permits at 10 a year each, but state law, in effect since February 2, 2025, bars caps on the number of ADUs. Ask the Building Division how it applies these rules today.

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

  1. Sec. VII.B.19.3.a Accessory Dwelling Unit, Requirements, Protective Zoning Bylaw, as amended through May 2025 (attested 09/03/2025), p. 93
    An ADU constructed within a single family dwelling or attached to a single family dwelling shall be permitted as a “By Right” use and shall meet all requirements of the Town of Chatham Protective Bylaw. No more than ten (10) By Right Permits shall be granted within a twelve (12) month period, January 1st to December 31st and the number of permits shall be counted according to projects approved.
  2. Sec. VII.B.19.3.b Accessory Dwelling Unit, Requirements, Protective Zoning Bylaw, as amended through May 2025 (attested 09/03/2025), p. 93
    A Special Permit from the Zoning Board of Appeals is required in the following instances and no more than ten (10) Special Permits shall be granted by the Special Permit Granting Authority for the construction of an ADU within a twelve (12) month period January 1st to December 31st and the number of permits shall be counted according to projects approved. i. An ADU is constructed as a detached accessory structure- not attached to a single family dwelling.
  3. Sec. VII.B.19.4.d Use and Dimensional Requirements, Protective Zoning Bylaw, as amended through May 2025 (attested 09/03/2025), p. 94
    The ADU shall contain no more than two bedrooms and contain no more than 900 square feet of habitable space. Once an ADU has been added to a single family dwelling or lot, the accessory dwelling unit shall not be enlarged.
  4. Sec. VII.B.19.4.e Use and Dimensional Requirements, Protective Zoning Bylaw, as amended through May 2025 (attested 09/03/2025), p. 94
    At least one (1) off street parking space in addition to that required for the principal single family dwelling is required for each ADU. All parking for ADUs shall be off street.
  5. Sec. VII.B.19.4.h Use and Dimensional Requirements, Protective Zoning Bylaw, as amended through May 2025 (attested 09/03/2025), p. 94
    Either the ADU or the principal dwelling shall be leased for a minimum of a twelve (12) consecutive months, with no subletting to occur. Both units are prohibited from any use as rental units on a monthly, weekly or daily basis including, but not limited to, seasonal rental and rental through vacation rental services and websites.
  6. Sec. VII.B.19.4.a Use and Dimensional Requirements, Protective Zoning Bylaw, as amended through May 2025 (attested 09/03/2025), p. 93
    Only one ADU is permitted for each lot.
  7. Sec. VII.B.19.4.g Use and Dimensional Requirements, Protective Zoning Bylaw, as amended through May 2025 (attested 09/03/2025), p. 94
    An ADU may not be sold or transferred separate and apart from the principal dwelling to which it is an accessory use.
  8. Sec. VII.B.19.4.k Use and Dimensional Requirements, Protective Zoning Bylaw, as amended through May 2025 (attested 09/03/2025), p. 94
    The ADU shall become the principal residence of the renter within 30 (thirty) days of occupancy of the ADU.
  9. Sec. VII.B.19.5 Monitoring, Protective Zoning Bylaw, as amended through May 2025 (attested 09/03/2025), p. 94
    The property owner shall be required to file a copy of the lease agreement for the rental unit with the Building Commissioner annually on or before January 31st, stating that either the principal dwelling or the accessory dwelling unit will be leased for a minimum of a twelve (12) consecutive months, with no subletting to occur.
This page summarizes Chatham'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 Community Development - Building Division (Building Commissioner) (508-945-5168) before you build. Spot an error?