Building an ADU in Maine: what state law allows

What state law lets you build now

Maine's ADU law, 30-A M.R.S. 4364-B, began as LD 2003 (Public Law 2021, chapter 672). It applies to every town from its "implementation date": January 1, 2024 for towns whose municipal officers can enact ordinances without a vote of residents, and July 1, 2024 for all other towns.

Under the version that applies to towns until July 1, 2027, a town must allow an ADU on the same lot as a single-family home "in any area in which residential uses are permitted, including as a conditional use," and "at least one accessory dwelling unit must be allowed on any lot where a single-family dwelling unit is the principal structure." You can build it:

• Within the existing home
• Attached to or sharing a wall with the home
• As a new building on the lot whose main purpose is the ADU

State law also says an ADU that was built without town approval "must be allowed" if it otherwise meets the town's and the state's ADU requirements. [1][2][3]

What towns can and can't require

State law limits how towns can regulate an ADU:

• Parking: an ADU "may not be subject to any additional parking requirements beyond the parking requirements of the single-family dwelling unit" on the lot.
• Setbacks: for an ADU inside or attached to the house, setbacks and dimensional requirements must be the same as for the house. An ADU in an existing accessory building or garage keeps the setbacks for that structure. Towns may be more generous.
• Density: a town must exempt the ADU from density requirements tied to the area where it is built.
• Nonconforming lots: an ADU is allowed on a lot that doesn't meet current zoning if it doesn't make the nonconformity worse.
• Size: an ADU "must meet a minimum size of 190 square feet." A town "may impose a maximum size."
• Growth caps: an ADU permit doesn't count toward a town's rate of growth ordinance.

Towns may also adopt less restrictive criteria in cases where they could grant a variance. Private deed restrictions and covenants that are stricter than state law still apply. [1][3]

Water, sewer and septic

Before the town can certify your ADU for occupancy, you must give it written proof that the ADU is connected to adequate water and wastewater service:

• Public sewer: proof of adequate service for the added flow and proof you paid for the connection.
• Septic: the system must be verified as adequate by the local plumbing inspector, and plans must be prepared by a licensed site evaluator.
• Public water: proof of adequate service and payment for the connection.
• Well: proof the water is potable.

The ADU rule is subject to Maine's minimum lot size law for subsurface wastewater (Title 12, chapter 423-A). [1][5]

Shoreland zones and flood areas

An ADU must meet state and local shoreland zoning rules. But a town "may not categorically prohibit accessory dwelling units in the shoreland zone" if they would otherwise meet those rules.

The ADU law doesn't apply to lots in the watershed of a water source in Lewiston or Auburn that supplies drinking water under a federal filtration waiver. [1]

Changes coming July 1, 2027

In 2025 the Legislature passed LD 1829 (Public Law 2025, chapter 385), and in 2026 it enacted LD 2173 (chapter 733, signed April 16, 2026), which the state Office of Community Affairs describes as "mostly refined and amended language from LD 1829" that "extended deadlines for local compliance for most sections." Under chapter 733, these ADU changes apply to all towns beginning July 1, 2027:

• ADUs on more lots: at least one ADU must be allowed where the principal structure is "a single-family dwelling unit or 2-unit or 3-unit residential structure."
• No owner-residency rule: a town "shall allow the construction or occupancy of an accessory dwelling unit on a lot even if the owner of the lot ... does not reside in a dwelling unit on that lot."
• Density: a town must exempt "one" ADU per lot from density requirements.
• No planning board: for areas where homes are allowed, a town may not require planning board approval for ADUs.
• Units per lot: at least 3 dwelling units per lot, "inclusive of accessory dwelling units," or at least 4 in a designated growth area or on a lot served by public water and public sewer.
• Flood and coastal areas: a town may exempt lots in coastal barrier areas, special flood hazard areas and coastal sand dune systems by ordinance.
• Parking in growth areas: under a separate 2025 law (chapter 374), whose start date chapter 733 also set at July 1, 2027, a town may not require more than one off-street parking space per dwelling unit for residential development in a designated growth area.

The Legislature's online statute pages already show this new wording, with notes on when it applies. Until July 1, 2027, ask your town which rules it is applying. [1][3][4][6][7]

Fire sprinklers

Under 25 M.R.S. 2463-B, fire sprinklers "are not required for an accessory dwelling unit unless the accessory dwelling unit is within or attached to a structure that contains, or will contain upon completion of construction, more than 2 dwelling units, including accessory dwelling units." Sprinklers may still be required in a mixed-use or nonresidential building, or where fire codes require them for other reasons. Chapter 733 (2026) added the "will contain" wording and these exceptions; the 2025 original in chapter 385 said "a structure of more than 2 dwelling units." [3][4][8]

Short-term rentals and local rules

The Office of Community Affairs says towns may still "REGULATE the maximum size of accessory dwelling units" and "REGULATE short-term rentals in their community," and may allow more housing than state law requires. Check your town's ordinance before renting an ADU by the night.

Each town writes its own ADU ordinance within these limits. For a city example, see our page on building an ADU in Portland, Maine. [9]

Where to get help

The Maine Office of Community Affairs runs the Housing Opportunity Program, which helps towns put these laws into effect and publishes guidance on them. It can be reached at housing.moca@maine.gov.

For the general process, see our guides to the ADU permit process, ADU costs, ADU financing and garage conversions. [10]

Maine laws to know

• LD 2003 (P.L. 2021, c. 672), 30-A M.R.S. 4364-B

  Requires towns to allow at least one ADU on lots with a single-family home and limits parking, setback, density and size rules for it. [1]

• P.L. 2023, c. 192

  Set the implementation dates (January 1 or July 1, 2024) and added the rule that an ADU built without town approval must be allowed if it meets the requirements. [1][2]

• LD 1829 (P.L. 2025, c. 385)

  Bars owner-residency requirements, extends ADUs to multi-unit lots and raises the minimum units per lot (for towns from July 1, 2027), and limits sprinkler rules for ADUs. [3][7]

• LD 2173 (P.L. 2025, c. 733, signed April 16, 2026)

  Refined LD 1829, limited the ADU rule to lots with single-family, 2-unit or 3-unit homes, and moved most deadlines to July 1, 2027. [4][7]

Questions people ask