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Can I build an ADU in Brunswick, Maine?

Brunswick, Cumberland County · Ordinances checked 2026-10-01 · Reviewed · 9 cited sections

Allowed with a permit

Yes. Brunswick allows one accessory dwelling unit per lot on single- or two-family residential lots, inside the house, attached, or in a detached building, without Development Review. The ADU can be no larger than 1,100 sq ft or 75% of the main home's floor area, whichever is less, and you need a permit backed by proof of adequate water and wastewater.

Where allowedLots primarily used as single- or two-family residential [2]
Number per lot1 ADU [3]
Max size1,100 sq ft or 75% of the largest dwelling unit's floor area, whichever is less [4]
Detached ADU heightNo taller than the principal structure (unless above a garage) [5]
Development ReviewNot required [2]
OwnershipMust be owned by the principal structure's owner; no condo [7]
Rental registrationADUs must register under the rental registry [8]

Where ADUs are allowed

Brunswick defines an ADU as a self-contained unit with living, cooking and bathroom facilities for one household, located within, attached to, or detached from a single- or two-family dwelling [1]. ADUs are allowed on lots primarily used as single- or two-family residential, in the main building or a detached structure, and they do not require Development Review or count toward density [2]. In the zoning use tables (Tables 3.2 and 3.3), ADUs are permitted in every growth and rural district that permits one- and two-family homes; the few districts that prohibit ADUs (GM7, GA, GI, GO, GN and RN) prohibit homes too.

Size, number and design

Only one ADU is allowed per lot; any further units must meet density and dimensional rules [3]. An ADU can be no larger than 1,100 square feet or 75% of the gross floor area of the largest dwelling unit, whichever is less; garage or carport space used by the ADU doesn't count [4]. A detached ADU can't be taller than the principal structure unless it sits above a garage and meets dimensional standards [5]. Where practical, the ADU's entrance should be on a different side of the building than the main home's.

Water, sewer and ownership

No ADU permit is issued until qualified professionals verify adequate, safe water supply and wastewater connection [6]. The ADU must be owned by the owner of the principal structure and cannot be a condominium unit [7]. We found no owner-occupancy requirement and no ADU-specific parking rule in the zoning text.

Renting it out

If you rent the ADU (long-term or short-term), it is a rental unit under Chapter 8 and must be registered annually with the Fire Department's Fire Prevention Office [8].

Shoreland

In the Shoreland Protection Overlay, the per-dwelling-unit minimums (30,000 sq ft of lot area and 150 ft of shore frontage next to tidal areas; 40,000 sq ft and 200 ft next to non-tidal areas) apply to ADUs too, notwithstanding Section 3.4.2.A [9]. In other words, a waterfront lot needs that much area and frontage for each unit, counting the ADU. That can rule out an ADU on smaller waterfront lots.

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

Next steps

  1. Confirm your lot is primarily single- or two-family residential and check for Shoreland Protection Overlay mapping with Planning & Development (207-725-6660).
  2. Size the unit at no more than 1,100 sq ft or 75% of the main home's floor area, whichever is less.
  3. Get written verification from qualified professionals of adequate water supply and wastewater (sewer or septic) capacity.
  4. Apply for a building permit with Codes Enforcement, including plans and the water/wastewater verification.
  5. If you will rent the unit, register it with the Fire Department's Fire Prevention Office under the rental registry.

Common questions

Can I build an ADU in Brunswick, Maine?
Yes. Brunswick allows one accessory dwelling unit per lot on single- or two-family residential lots, inside the house, attached, or in a detached building, without Development Review. You need a permit backed by proof of adequate water and wastewater.
How big can an ADU be in Brunswick, Maine?
No larger than 1,100 sq ft or 75% of the gross floor area of the largest dwelling unit, whichever is less. A detached ADU can't be taller than the principal structure unless it sits above a garage and meets dimensional standards.
Do I have to live on the property to have an ADU in Brunswick, Maine?
We found no owner-occupancy requirement and no ADU-specific parking rule in the zoning text. The ADU must be owned by the owner of the principal structure and can't be a condominium unit.
Do I need to register a rental ADU in Brunswick, Maine?
Yes. If you rent the ADU, long-term or short-term, it is a rental unit under Chapter 8 and must be registered annually with the Fire Department's Fire Prevention Office.
Can I build an ADU on a waterfront lot in Brunswick, Maine?
It may be ruled out on smaller lots. In the Shoreland Protection Overlay, the per-dwelling-unit minimums, 30,000 sq ft and 150 ft of shore frontage next to tidal areas or 40,000 sq ft and 200 ft next to non-tidal areas, apply to ADUs too.

Maine's statewide ADU law

Under 30-A M.R.S. §4364-B (first passed as LD 2003 in 2022), a town must allow at least one accessory dwelling unit (ADU) on a lot with a single-family home anywhere homes are allowed. Towns had to comply by 2024.

Rules in effect now:

  • It can be inside the house, attached, or a new detached building.
  • One ADU per lot is exempt from density limits.
  • No extra parking can be required beyond what the house already needs.
  • Setbacks are the same as for the house.
  • Minimum size: the state building codes board set 160 sq ft in 2025. Towns may set a maximum size.
  • Before you can move in, you must show the ADU has adequate water and septic/sewer.
  • In the shoreland zone ADUs must meet shoreland rules, but towns can't ban them there outright.
  • Sprinklers aren't required unless the ADU is in or attached to a building with more than 2 units.

Coming July 1, 2027 (LD 1829 of 2025, pushed back by LD 2173 in 2026): towns can no longer require the owner to live on the property, ADUs must also be allowed next to 2- and 3-unit buildings, and towns can't require planning board approval for an ADU. Towns may still regulate short-term rentals in ADUs. Private covenants and HOA rules can be stricter.

State sources: 30-A M.R.S. §4364-B (Accessory dwelling units) · 30-A M.R.S. §4364 (implementation dates) · 25 M.R.S. §2463-B (sprinklers in ADUs) · LD 2173 (P.L. 2025, c. 733) status and chaptered law · Maine Office of Community Affairs, Housing Legislation Guidance (March 2026)

What the ordinance says

Exact text from Brunswick's published ordinances. Section numbers are as printed in the source.

  1. Sec. 1.7.2 Definitions, Accessory Dwelling Unit (ADU), Zoning Ordinance (Appendix A, Municipal Code)
    A self-contained dwelling unit, featuring living, cooking and bathroom facilities, designed for and occupied by no more than one (1) household and located within, attached to, or detached from a single- or two-family dwelling.
  2. Sec. 3.4.2.A(1)-(3), Zoning Ordinance (Appendix A, Municipal Code)
    An ADU does not require Development Review. (2) The creation of an ADU shall not be counted for density purposes. (3) ADUs are allowed on lots primarily used as single or two family residential in either a principal or detached structure.
  3. Sec. 3.4.2.A(4), Zoning Ordinance (Appendix A, Municipal Code)
    Only one (1) ADU is permitted per lot; additional dwelling units shall be subject to density and dimensional requirements.
  4. Sec. 3.4.2.A(6), Zoning Ordinance (Appendix A, Municipal Code)
    The ADU shall not be greater than 1,100 square feet or 75 percent of the gross floor area of the largest dwelling unit, whichever is less. Any garage or carport floor area utilized by ADU will not count towards the total ADU square footage.
  5. Sec. 3.4.2.A(7), Zoning Ordinance (Appendix A, Municipal Code)
    Detached ADUs shall not exceed the maximum height of the principal structure, unless constructed above a garage and all applicable dimensional standards are met.
  6. Sec. 3.4.2.A(8), Zoning Ordinance (Appendix A, Municipal Code)
    No permit for an ADU may be issued until the applicant has provided written verification from qualified professionals that the accessory dwelling (a) will have adequate and safe water supply; and (b) will have adequate connection to wastewater services, as required by 30-A M.R.S.A. § 4364-B(7), as amended.
  7. Sec. 3.4.2.A(9), Zoning Ordinance (Appendix A, Municipal Code)
    An ADU must be owned by the owner of the principal structure and may not be a condominium unit.
  8. Sec. 8-102 Definitions, Rental Unit, Chapter 8 Art. III Registration of Rental Property (clean adopted version), p. 1
    Short term rents and ADUs are subject to this ordinance.
  9. Table 4.2.5.F(1) (Shoreland Protection Overlay), note [3], Zoning Ordinance (Appendix A, Municipal Code)
    [3] Notwithstanding Section 3.4.2.A., these restrictions apply to accessory dwelling units in the shoreland protection overlay.
This page summarizes Brunswick's posted ordinances as of 2026-10-01. Ordinances are amended, and individual lots can have overlays, subdivision covenants, or HOA rules that are stricter. Confirm with Planning & Development Department (Codes Enforcement) (207-725-6660) before you build. Spot an error?