Maine ADU Rules: LD 2003, LD 1829 and What Towns Must Allow

Under Maine's ADU rules, every municipality must allow at least one accessory dwelling unit on any lot where a single-family house is the principal structure, attached, inside the house or detached, in any area where homes are allowed. The rules are in 30-A M.R.S. § 4364-B, created by LD 2003 in 2022 and widened by LD 1829 in 2025: towns cannot require the owner to live on site, cannot add parking beyond what the house needs, and must allow a unit as small as 190 sq ft. A 2026 law, LD 2173, adjusted the rules again. Law current as of October 2026.

This guide goes through the statute section by section, the deadlines (which LD 2173 moved again in 2026), sprinklers, septic, and an 800 sq ft two-bedroom ADU plan. For how Maine compares with other states, see our state-by-state ADU law guide.

From LD 2003 to LD 1829

• LD 2003 (Public Law 2021, chapter 672), passed in April 2022, created § 4364-B and required towns to allow one ADU on lots with a single-family home. A 2023 follow-up (P.L. 2023, c. 192) set the implementation dates: January 1, 2024 for municipalities whose officers can enact ordinances without a town vote, and July 1, 2024 for all others.
• LD 1829 (P.L. 2025, c. 385), signed June 20, 2025, rewrote parts of § 4364-B: ADUs must be allowed with 2- or 3-unit residential structures too, owner occupancy can no longer be required, and the former limits on how ADUs could be added were loosened.
• LD 2173, signed April 16, 2026, refined LD 1829, returned some authority to municipalities and extended most local compliance deadlines, with July 1, 2027 as the default where no other date is set, according to the Maine Office of Community Affairs (MOCA).

What the Maine ADU law requires

Two points are often misread. First, the town can still cap the size: § 4364-B(6) sets only the minimum (190 sq ft) and expressly lets a municipality impose a maximum. Second, shoreland zoning still applies, but a town may not ban ADUs outright in the shoreland zone [5]. The statute does not apply to lots in the watershed of the Lewiston and Auburn water supply [1-B]. LD 2173 also lets towns exempt coastal barrier resource systems, special flood hazard areas and coastal sand dune systems when their ordinances identify them [8-D].

Deadlines: when your town has to comply

The original ADU rules have applied since 2024 (January 1 or July 1, depending on how the town adopts ordinances). The LD 1829 changes came with their own compliance dates starting in 2026, and LD 2173 pushed most of them later, to July 1, 2027 where the law gives no other date. Local reporting has quoted different dates for different kinds of towns, which reflects that these dates moved twice. In practice: if your town's ordinance is stricter than § 4364-B, ask the code enforcement officer which version applies to an application filed today.

Sprinklers and the building code

LD 1829 also changed Maine's fire-suppression rules for small housing. MOCA summarizes it this way: fire sprinklers are not required in ADUs unless the ADU is attached to a structure with more than two units or is in a mixed-use building. A detached ADU or one added to a single-family house is therefore outside the sprinkler requirement, though you can still choose to install one.

The rest of construction follows the Maine Uniform Building and Energy Code (MUBEC), which is based on the International Residential Code. An attached ADU is a second dwelling unit, so expect a fire-resistance-rated separation from the main house and smoke and CO alarms. A detached ADU is reviewed like a small house: habitable rooms of at least 70 sq ft and 7 ft wide (2021 IRC R304) and an escape opening in each bedroom (2021 IRC R310). For the window sizes, see legal bedroom requirements.

An 800 sq ft two-bedroom ADU

FloorGen's 800 sq ft 2-bed ADU template is a detached cottage 40 × 20 ft to the outside of its walls (800.1 sq ft), with 736.3 sq ft inside the exterior walls and 700.5 sq ft of net room area: two bedrooms at one end, a bath and laundry in the middle, and an open kitchen, dining and living room at the other.

Whether it is allowed as drawn depends on your town's maximum size, which the statute leaves to the town. If the local cap is lower, the plan shrinks easily: drop the dining room into the living room and shorten the cottage, or turn it into a one-bedroom. At the other extreme, the 190 sq ft minimum is about the size of this plan's living room alone, so almost any real ADU clears it.

Septic matters more than zoning on many Maine lots. Subsurface systems are sized by bedrooms, and § 4364-B(7) requires a local plumbing inspector to verify that the system is adequate before the ADU can be occupied. Two bedrooms added to a three-bedroom house can mean a new or enlarged system, so price the septic work before the cottage.

Before you apply

1. Read your town's ADU ordinance and compare it with § 4364-B; where the town is stricter on owner occupancy, parking or density, the statute generally controls once its deadline has passed.
2. Check private covenants on your deed; the statute does not override them.
3. Check shoreland and flood zones, and the septic or sewer capacity.
4. Draw a site plan and floor plan; our guide to floor plans for a building permit lists what to show.

FloorGen's IRC profile checks room areas and widths, ceiling heights, bedroom escape windows, stairs and door swings. It does not check Maine zoning, shoreland rules, septic sizing or MUBEC. For how other New England states handle ADUs, see the Massachusetts ADU law, which caps protected ADUs at 900 sq ft. Further south, New York City's City of Yes ADU rules allow units of up to 800 sq ft.