In plain words

Maine has a statewide ADU law. An ADU, or accessory dwelling unit, is a small, complete second home on the same lot as a house. It has its own kitchen, bathroom and sleeping space. Since 2024, almost every Maine town must allow at least one ADU with a single-family home. This applies anywhere housing is allowed. The ADU can be inside the house, attached to it, or a new building in the yard. Towns cannot demand extra parking for it. For an ADU inside or attached to the house, setbacks must match the house's. Setbacks are the required distances from property lines. The state sets a small minimum size, and towns may set a maximum. You must show you have enough water and septic or sewer capacity. Since September 2025, towns cannot require fire sprinklers in most ADUs. The main exceptions are an ADU in or attached to a building with more than two homes or with a business, or where fire codes call for sprinklers. More changes start on July 1, 2027. From then on, towns cannot require 'owner occupancy,' meaning the owner must live on the property. They also cannot send ADUs to the planning board, a volunteer board that holds public reviews. ADUs will also be allowed next to small two- and three-unit buildings. Until that date, your town may still have owner-occupancy or planning board rules. The law does not cover the unorganized territories run by the Land Use Planning Commission. Whether you can build depends on your lot, local rules, shoreland zoning and your septic system.

The rules at a glance

State ADU lawYes
Where the state rule appliesAll municipalities that regulate land use under Title 30-A (cities, towns and plantations with local zoning); not the unorganized territories, plantations and townships overseen by the Land Use Planning Commission (LUPC)
Approved without a hearing?Partly. Some ADUs or some places get staff approval; others can still need a hearing.
Kinds of ADU the state protectsinternal, attached, detached
ADUs per lotAt least 1 ADU per lot with a single-family home (from July 1, 2027, also with a 2- or 3-unit building); towns may allow more. From July 1, 2027, a separate rule makes at least 3 homes per residential lot a permitted use (4 if the lot is in a designated growth area or has public water and sewer), and an ADU counts toward that number.
Owner living on siteTowns may require the owner to live on the property.
ParkingAn ADU may not be required to have any parking beyond what the single-family home already needs.
SizeMinimum size is set by the state building code board: the statute says 190 sq ft unless the board adopts a different minimum, and state guidance says the board set 160 sq ft in March 2025. Towns may set a maximum size.
Lot size and setbacksFor an ADU inside or attached to the house, setbacks and other dimensional rules must be the same as for the house; an ADU in a garage or accessory building that existed by the implementation date uses that building's setbacks. Towns may be more lenient. An ADU is exempt from density limits (one per lot from July 1, 2027). An ADU is allowed on a nonconforming lot if it does not increase the nonconformity. ADUs must meet shoreland zoning, but towns cannot ban them outright in the shoreland zone. The owner must give written proof of adequate water and wastewater service.
Short-term rentalsState law does not set short-term rental rules for ADUs; state guidance says towns may regulate short-term rentals, but from July 1, 2027 they cannot do it through owner-occupancy requirements.
Can towns opt out?No

What to do next

  1. Call your town's code enforcement officer and ask for the current ADU ordinance and whether it has been updated for the July 1, 2027 changes.
  2. If your land is in an unorganized territory or plantation, check with the Land Use Planning Commission instead (207-287-2631).
  3. Get written confirmation of water and wastewater capacity; a local plumbing inspector's sign-off on the septic or sewer is what the law looks for.
  4. If your lot is near a lake, river, wetland or the coast, check shoreland zoning and flood maps before you design.
  5. Ask your bank and regional council about ADU financing; in Bath, Brunswick and Rockland the ADU Boost pilot offers up to $10,000.

Grants, loans and tax breaks

  • ADU Boost Program (Bath, Brunswick and Rockland pilot): Regional pilot run by the Midcoast Council of Governments and Bangor Savings Bank, funded by the State of Maine's Housing Opportunity Grant Program. Homeowners who finance an ADU through the bank can get a grant equal to 10 percent of qualified costs, up to $10,000, first come first served. The unit cannot be sold as a condo or used as a short-term rental while the financing is in place. Program page

What your town still decides

Towns must allow the state minimum but run their own permit process, usually through the code enforcement officer. Until July 1, 2027, towns may still require planning board review and an owner-occupancy rule; after that date both are barred, though site plan and historic review by staff remain allowed. Towns can always be more generous, for example allowing two ADUs. From July 1, 2027 towns may exempt mapped coastal barrier, special flood hazard and coastal sand dune areas. LUPC areas follow separate rules under Title 12. The Maine Office of Community Affairs Housing Opportunity Program gives towns technical help and one-time payments and grants to update ordinances, and said in March 2026 that it would amend its zoning rule once LD 2173 passed.

Examples of local rules

  • Portland: Since a December 2020 ordinance, Portland can allow up to two ADUs on any residential property. They can be inside the main building, in a legally existing detached building, or in a new building or addition, under Chapter 14, Sec. 6.6.2.A. A building permit is required, and the city's January 2025 FAQ warns a new sewer line or sprinklers may be needed; the state sprinkler limit took effect later, in September 2025. Source
  • Bangor: Bangor allows one ADU per lot that has a single-family home, duplex or triplex, approved by the code enforcement officer. It must measure at least 160 sq ft, and its cap is the smaller of 1,000 sq ft or half the main home. ADUs have no minimum lot area, density or parking requirement; the section was last amended April 13, 2026. Source
  • South Portland: ADUs may be detached and larger than under older rules, with no extra parking required. A zoning permit costs $500 and takes about 3 to 4 weeks, followed by a building permit; standards are in city code Sections 27-1576 to 27-1579. Source

What we’re still confirming

  • Maine's ban on local owner-occupancy rules for ADUs doesn't apply to towns until July 1, 2027, even though March 2026 state guidance described it as already in effect.
  • Whether a recent sprinkler clarification in state law applies to towns now or only from July 1, 2027 is unclear.

Laws, bills and citations

The laws behind this (4)
  • An Act to Update the Laws Regarding Housing Developments and Accessory Dwelling Units (LD 2173, P.L. 2025, ch. 733 (132nd Legislature, 2nd Regular Session), amending 25 MRSA § 2463-B and 30-A MRSA §§ 4364-A, 4364-B, signed April 16, 2026, in effect July 1, 2027). A fix-up of the 2025 housing law. It moves the start date for the 2025 zoning changes to July 1, 2027 for every town, limits the ADU requirement to lots with a single-family home or a 2- or 3-unit building, adds an explicit rule that towns cannot require planning board approval for ADUs, and lets towns exempt coastal barrier, flood hazard and sand dune areas. It also says sprinklers are still required when an ADU is in or attached to a mixed-use or nonresidential building or when fire codes otherwise call for them. Read it
  • An Act to Build Housing for Maine Families and Attract Workers to Maine Businesses by Amending the Laws Governing Housing Density (LD 1829, P.L. 2025, ch. 385 (132nd Legislature, 1st Special Session), amending 25 MRSA § 2463-B, 30-A MRSA §§ 4301(1-C), 4364-A, 4364-B and others, signed June 20, 2025, in effect September 24, 2025). Bars towns from requiring fire sprinklers in an ADU unless it is in or attached to a building with more than two units. Starting July 1, 2027, towns may not require the owner to live on the lot and must allow ADUs next to small multi-unit buildings. It also tightens two 2022 rules: towns 'must' (not just 'may') allow an ADU on a nonconforming lot that does not worsen the nonconformity, and the density exemption covers one ADU per lot. Read it
  • An Act to Clarify Statewide Laws Regarding Affordable Housing and Accessory Dwelling Units (LD 1706, P.L. 2023, ch. 192, amending 30-A MRSA §§ 4364, 4364-A, 4364-B, signed June 16, 2023, in effect January 1, 2024). Emergency law that pushed back the date towns had to comply with the 2022 ADU mandate and clarified its wording. After these dates, every covered town had to allow at least one ADU with a single-family home. Read it
  • Original statewide ADU mandate (part of the 2022 housing opportunity law) (LD 2003, P.L. 2021, ch. 672, enacting 30-A MRSA § 4364-B, signed April 27, 2022, in effect July 1, 2023). Requires towns to allow at least one ADU on any lot where a single-family home is the main building, anywhere housing is allowed. The ADU can be inside the house, attached, or a new detached building. Together with the 2023 amendments, the section also bars extra parking requirements, uses the house's setbacks for inside or attached units, sets a state minimum size, and lets towns set a maximum. Read it
Proposed and failed bills (1). None of these is law.
  • LD 1272 (132nd Legislature, 2025): died as of October 3, 2026. 'An Act to Address the Housing Crisis by Reducing Barriers to Building More Accessory Dwelling Units.' The committee voted it Ought Not to Pass under Joint Rule 310 on May 27, 2025, the same session LD 1829 passed. Bill page

Sources

  1. LD 2173, P.L. 2025, ch. 733 (132nd Legislature, 2nd Regular Session), amending 25 MRSA § 2463-B and 30-A MRSA §§ 4364-A, 4364-B · official source
  2. LD 1829, P.L. 2025, ch. 385 (132nd Legislature, 1st Special Session), amending 25 MRSA § 2463-B, 30-A MRSA §§ 4301(1-C), 4364-A, 4364-B and others · official source
  3. LD 1706, P.L. 2023, ch. 192, amending 30-A MRSA §§ 4364, 4364-A, 4364-B · official source
  4. LD 2003, P.L. 2021, ch. 672, enacting 30-A MRSA § 4364-B · official source
  5. 30-A MRSA § 4364-B Accessory dwelling units · official source · checked 2026-10-03
  6. LD 2173 status page (132nd Legislature) · official source · checked 2026-10-03
  7. OPLA: Housing and Economic Development Committee, disposition of bills and law summaries, 132nd 2nd Regular Session (May 2026) · official source · checked 2026-10-03
  8. Maine Office of Community Affairs: Housing Legislation Guidance (March 2026) · official source · checked 2026-10-03
  9. Maine Office of Community Affairs: LD 1829 & LD 2173 Overview slide deck (August 2026) · official source · checked 2026-10-03
  10. LD 1706 (131st Legislature) bill text · official source · checked 2026-10-03
  11. LD 1272 status page (132nd Legislature) · official source · checked 2026-10-03
  12. Town of Brunswick: ADU Boost Program · official source · checked 2026-10-03
  13. City of Bangor Code § 165-31.2 Accessory dwelling units (eCode360) · official source · checked 2026-10-03
  14. City of Portland Zoning FAQ (January 10, 2025) · official source · checked 2026-10-03
  15. City of South Portland: Accessory Dwelling Units · official source · checked 2026-10-03
  16. Bangor Daily News: Maine lawmakers advance bill to allow more ADUs, greater housing density (Mar 20, 2026) · checked 2026-10-03
  17. Citizen Portal: Committee shifts implementation dates for ADU and housing-development changes (Mar 31, 2026) · checked 2026-10-03

This is general information, not legal advice. Your town, village, city or county decides what can be built on your property. If something here looks out of date, email the team.