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ADU rules in Maine

Representative cities: Portland, Lewiston, Bangor.

Verify locally. Maine enforces the Maine Uniform Building & Energy Code (MUBEC) — the 2021 IRC with Maine amendments (effective April 7, 2025). MUBEC is mandatory only in municipalities with ≥4,000 residents; smaller towns may opt in. Permits are issued by the local Code Enforcement Officer; electrical permits are handled by the State of Maine. ADU rules below are drawn from official sources — always confirm with the issuing building department before designing or building.

ADU permit rule

Permit requiredGenerally allowedcity authority
Fee basis
Valuation-based; set by the issuing municipality (no statewide flat fee).
Typical timeline
Plan review typically 2–4 weeks in staffed cities; over-the-counter where pre-approved.
Setback
Per local zoning; shoreland-zone lots have stricter setback rules statewide.
Area threshold
No statewide ADU mandate; size limits set by local ordinance.
Exempt if
None — an ADU (in-law apartment, detached cottage) always requires a building permit where the town enforces MUBEC.
Go / no-go
allowed
Maine has no statewide ADU law. ADUs are allowed only where the local ordinance permits them (e.g., Portland). Verify with your Code Enforcement Officer — many small towns have no ADU provision at all.
ADU official sources: Maine DPS — MUBEC Rules (Residential Building Code Ch. 5) · Town of Durham — Code Enforcement / Permit Exemptions
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Frequently asked questions

Do I need a permit to build an ADU in Maine?

Per Maine rules, an ADU is "yes" for a permit and the authority is "city". Maine has no statewide ADU law. ADUs are allowed only where the local ordinance permits them (e.g., Portland). Verify with your Code Enforcement Officer — many small towns have no ADU provision at all. Always confirm with the issuing building department.

How big can an ADU be in Maine?

Area threshold: No statewide ADU mandate; size limits set by local ordinance.

What are the ADU setbacks in Maine?

Setback: Per local zoning; shoreland-zone lots have stricter setback rules statewide.

How much does an ADU permit cost in Maine?

Fee basis: Valuation-based; set by the issuing municipality (no statewide flat fee).

State-wide sources: Maine DPS — MUBEC Rules and Laws · Maine Office of Energy — Building & Energy Codes · Town of Durham — Code Enforcement / Permit Exemptions

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