ADU Guide · Verified 2026
Building an ADU in Maine
Maine requires every municipality to allow at least one ADU on any lot where a single-family dwelling is the principal structure, and a 2025 amendment extended that protection to lots with multi-unit structures too. Some towns still have outdated local ordinances on the books that contradict the current state law, a real friction point worth checking before assuming the town's own code is current.
Quick Facts
Statewide floors set by LD 2003 (2022), 30-A M.R.S. 4364-B; amended by LD 1829 (2025). Your city may be more generous but cannot go below them.
The Three Types of ADU
A law that got expanded again in 2025
LD 2003 originally covered lots where a single-family dwelling was the principal structure. LD 1829, passed in 2025, extended that protection to lots with multi-unit structures too, a real, recent widening of who the law actually covers.
Some towns never updated their own code
Gorham and Westbrook both still have local ordinance language requiring owner-occupancy for an ADU, text that predates LD 2003 and directly contradicts the current statewide standard, which eliminated any owner-occupancy requirement. A genuine, documented example of local code lagging behind state law.
An existing unpermitted unit isn't automatically legal
LD 2003 makes future ADUs easier to build, but it does not retroactively legalize an ADU that was built without a permit before the law existed; the owner still needs to go through the standard permitting process to bring it into compliance.
What people get wrong
The most common mistake is trusting a town's own zoning code without checking it against current state law. Gorham and Westbrook both show real examples where the local text is simply out of date.
From Idea to Move-In: The Timeline
Who Administers It
Building It Yourself? Here's What You'll Need
Planning an ADU? Tools homeowners use in the design and layout phase:
As an Amazon Associate, we earn from qualifying purchases.
Check Your Local Rules
Your city may allow more than the state floor
State law sets the minimum; many Maine cities and counties are more generous. Browse the full directory of 16 counties and every city in Maine for local rules, fees, and building department contact details.
Browse the full Maine directory →Not sure if your project needs a building permit, a zoning approval, or both? See our guide to building permits vs. zoning approval.
Official Sources
ADU law changes frequently and local ordinances add detail. This guide is general information, not legal advice, always confirm with your local building department. Reviewed by the StateDataIndex Editorial Team · Updated September 2026.