Maine ADU permit requirements
Last reviewed: June 20, 2026
Quick answer
Permitting an accessory dwelling unit (ADU) in Maine means pulling a building permit from the local authority that issues it. Maine has a statewide ADU law that sets baseline standards local agencies must follow. ArchPermit tracks 14 source-cited requirements (12 required, the rest conditional) for this jurisdiction. These requirements were last reviewed June 20, 2026; confirm the exact forms and fees with the local permitting authority.
Maine has a statewide ADU law. This baseline reflects that law plus the universal new-dwelling deliverables. Your local city or county still administers the permit and may add its own forms and fees, so confirm the exact checklist with them.
Maine's LD 2003 (2022, 30-A M.R.S. § 4364-B) requires every municipality to allow at least one ADU on any lot with a single-family principal dwelling, with no minimum size below 190 sq ft, no added parking beyond the single-family requirement, and no owner-occupancy requirement. MUBEC enforcement is mandatory for municipalities of 4,000+.
- Adopted building code compliancerequired
The ADU must comply with Maine's adopted residential building code: Maine Uniform Building & Energy Code (MUBEC, 2021 IRC). Confirm the in-force edition with the local AHJ.
source · reviewed 2026-06-20 - At least one ADU per single-family lotrequired
A municipality must allow ≥1 ADU on any lot where a single-family dwelling is the principal structure.
source · reviewed 2026-06-20 - Building cross-sectionsrequired
Cross-sections through the structure showing ceiling heights, wall/roof assemblies, insulation, and fire/sound-rated assemblies where required.
source · reviewed 2026-06-20 - Egress & life-safetyrequired
Emergency escape and rescue openings (egress windows), smoke and carbon-monoxide alarms, and required exits per the adopted residential code.
source · reviewed 2026-06-20 - Energy code compliancerequired
Energy-code compliance documentation for the conditioned space. Maine: MUBEC baseline: 2021 IECC.
source · reviewed 2026-06-20 - Exterior elevationsrequired
Elevations of each affected building face showing height, exterior materials, and openings; existing vs. proposed where exterior changes occur.
source · reviewed 2026-06-20 - Floor plansrequired
Dimensioned floor plans of the ADU (and affected areas of the primary dwelling) showing rooms, uses, door/window locations, egress, and fixture/equipment locations.
source · reviewed 2026-06-20 - Foundation planrequired
Foundation plan with footing/stem-wall sizes and details (or a cited site-specific design), per the adopted residential code.
source · reviewed 2026-06-20 - Framing / structural plansrequired
Floor, wall, and roof framing showing member sizes, spans, spacing, connections, and lateral bracing; structural calculations where the framing is non-conventional.
source · reviewed 2026-06-20 - Local zoning / setback compliancerequiredverify locally
Compliance with the local jurisdiction's zoning: permitted use, setbacks, height, lot coverage, and any ADU-specific standards. These are set locally. Confirm with the permitting agency.
source · reviewed 2026-06-20 - Mechanical / plumbing / electricalrequired
HVAC, plumbing fixtures and lines, and electrical (panels, circuits, outlets, lighting, smoke/CO alarms) for the new unit.
source · reviewed 2026-06-20 - Site / plot plan (to scale)required
Scaled site plan showing property lines, setbacks, existing structures, the proposed ADU footprint with dimensions to property lines, driveways/parking, and utility locations.
source · reviewed 2026-06-20 - Min-size floor 190 sf; no extra parkingconditional
Minimum allowable ADU size is 190 sq ft; no parking beyond the single-family requirement. Municipalities may set a maximum size.
source · reviewed 2026-06-20 - Owner-occupancy may not be requiredconditional
An ADU must be allowed even if the owner does not reside on the lot.
source · reviewed 2026-06-20
Maine ADU permit questions
- Do I need a permit to build an ADU in Maine?
- Yes. An accessory dwelling unit (ADU) in Maine requires a building permit, and Maine has a statewide ADU law that sets baseline standards local agencies must follow. ArchPermit tracks 14 cited requirements for this jurisdiction (12 required, the rest conditional). Always confirm the exact forms, fees, and code edition with the local permitting authority that issues the permit.
- What does an ADU permit package require in Maine?
- Core required items include: Adopted building code compliance; At least one ADU per single-family lot; Building cross-sections; Egress & life-safety; Energy code compliance; Exterior elevations, plus 6 more. Each requirement links to its authoritative source, and nothing is permit-ready until an independent licensed professional reviews and seals it.