Maine state law: statewide ADU mandate under 30-A MRS § 4364-A (LD 2003) preempts local single-family-zone ADU bans

30-A MRS § 4364-A (Accessory dwelling units) was enacted by LD 2003 / PL 2021 ch. 672 ('An Act To Implement the Recommendations of the Commission To Increase Housing Opportunities in Maine by Studying Zoning and Land Use Restrictions'). The statute provides that every Maine municipality SHALL allow at least one ADU as an accessory use to a single-family dwelling in any zone in which a single-family dwelling is permitted, subject to the same dimensional and density requirements as the single-family dwelling. Key preemptions: a municipality MAY NOT require owner-occupancy of either the primary dwelling or the ADU; MAY NOT require more than one additional off-street parking space per ADU; MAY NOT impose minimum-lot-size, minimum-floor-area, or design standards that effectively prohibit ADUs; and MAY NOT apply more restrictive setback or design standards than for the primary dwelling. Internal, attached, and detached ADUs are all within scope. The mandate took effect through phased implementation in 2024 (January 1, 2024 for some provisions; July 1, 2024 for others). The companion statute 30-A MRS § 4364 (Affordable-housing density bonus) requires municipalities to allow affordable-housing developments (≤80% AMI rental, ≤120% AMI ownership) at 2.5 times base multifamily density where multifamily is otherwise allowed. The mandatory Shoreland Zone (38 MRS § 435 et seq.), the Maine Uniform Building and Energy Code (25 MRS § 2451), the National Flood Insurance Program ordinance, deed restrictions, and historic-district overlay all continue to apply.

A municipal zoning ordinance that conflicts with 30-A MRS § 4364-A is preempted and unenforceable. An aggrieved property owner may seek a declaratory judgment in Superior Court or appeal a permit denial to the Board of Appeals and then to court under 30-A MRS § 2691 and Rule 80B.