Maine's LD 2003 (Public Law 2021, chapter 672), codified at 30-A M.R.S. section 4364-B, requires every municipality to allow at least one accessory dwelling unit on any lot whose main building is a single-family home or a 2- or 3-unit building. Municipal compliance dates are January 1, 2024 and July 1, 2024, and the minimum ADU size is 190 square feet.
Last verified: October 18, 2026 against the Maine Revisor's Statutes at 30-A M.R.S. sections 4364, 4364-A and 4364-B (Revisor data as of October 5, 2026) and the Maine DECD Housing Opportunity Program update.
An accessory dwelling unit, or ADU, is a second, smaller home on a lot that already has a house. Maine law makes ADUs a statewide right through LD 2003, and this page explains what that law allows, the statute that controls it, and the municipal compliance dates now in force.
What does Maine ADU law allow?
Maine ADU law is set by LD 2003, “An Act To Implement the Recommendations of the Commission To Increase Housing Opportunities in Maine by Studying Zoning and Land Use Restrictions.” LD 2003 was enacted by the 130th Legislature, Second Regular Session, and codified as Public Law 2021, chapter 672. The ADU rule itself lives at 30-A M.R.S. section 4364-B, not section 4364-A. Section 4364-A is a separate rule that governs broader multi-unit residential density.
The core rule is a by-right mandate, meaning the use is allowed without a special discretionary permit. Under 30-A M.R.S. section 4364-B, subsection 3, at least one accessory dwelling unit “must be allowed on any lot where the principal structure is a single-family dwelling unit or 2-unit or 3-unit residential structure.” That wording is newer than most guides you will find: Public Law 2025, chapter 733 widened the rule from single-family lots to lots with 2- and 3-unit buildings. A municipality must allow the ADU wherever residential uses are permitted, including as a conditional use, which is a use a town allows only if it meets stated conditions.
The permitting process cannot run through the planning board. Under section 4364-A, subsection 5-A, a municipality may not require planning board approval for accessory dwelling units in any area where residential uses are allowed. (That rule used to sit in section 4364-B itself; chapter 733 repealed the old paragraph there, and the planning board bar now lives in 4364-A.) An ADU may be built within an existing dwelling unit, attached to or sharing a wall with the main house or a multi-unit building, or as a new structure on the lot. An ADU must also be allowed on a nonconforming lot, meaning a lot that does not meet current dimensional rules, so long as it does not further increase the nonconformity.
Maine ADU rules at a glance
| Rule | Maine standard | Statute | Source |
|---|---|---|---|
| At least one ADU per lot | Required on any lot whose principal structure is a single-family home or a 2- or 3-unit building | 30-A M.R.S. section 4364-B, sub-3, para. A | Maine Revisor, section 4364-B |
| Owner occupancy | Cannot be required; an ADU is allowed even if the owner does not reside on the lot | 30-A M.R.S. section 4364-B, sub-4-E | Maine Revisor, section 4364-B |
| Parking | No additional parking beyond what the single-family dwelling requires | 30-A M.R.S. section 4364-B, sub-4-C | Maine Revisor, section 4364-B |
| Minimum size | Must meet a minimum of 190 square feet; a town may set a maximum | 30-A M.R.S. section 4364-B, sub-6 | Maine Revisor, section 4364-B |
| Density | One ADU per lot is exempt from density requirements or calculations | 30-A M.R.S. section 4364-B, sub-4-A | Maine Revisor, section 4364-B |
| Setbacks and dimensions | Must be the same as those for the single-family dwelling | 30-A M.R.S. section 4364-B, sub-4-B | Maine Revisor, section 4364-B |
| Rate-of-growth caps | An ADU does not count toward a municipality’s rate of growth ordinance | 30-A M.R.S. section 4364-B, sub-9 | Maine Revisor, section 4364-B |
| Planning board | No planning board approval may be required for an ADU | 30-A M.R.S. section 4364-A, sub-5-A | Maine Revisor, section 4364-A |
| Hazard-area opt-out | A town may exempt lots in a coastal barrier, special flood hazard area or coastal sand dune system | 30-A M.R.S. section 4364-B, sub-8, para. D | Maine Revisor, section 4364-B |
How big can an ADU be in Maine?
Maine sets a statewide floor but lets towns set a ceiling. Under 30-A M.R.S. section 4364-B, subsection 6, an ADU “must meet a minimum size of 190 square feet,” and “A municipality may impose a maximum size.” So no Maine town can forbid an ADU for being too small if it is at least 190 square feet, but each town may cap how large an ADU can be within its own ordinance.
Does Maine require owner occupancy for an ADU?
No. Maine prohibits an owner-occupancy requirement for an ADU. Under 30-A M.R.S. section 4364-B, subsection 4-E, a municipality “shall allow the construction or occupancy of an accessory dwelling unit on a lot even if the owner of the lot … does not reside.” A town cannot make you live on the property in order to build or rent out an ADU.
Maine also bars a town from adding parking. Under subsection 4-C, an ADU “may not be subject to any additional parking requirements beyond the parking requirements of the single-family dwelling unit.”
What can local governments still regulate?
Municipalities keep some authority, but only within the limits the statute sets. Section 4364-B, subsection 3, states that “With respect to accessory dwelling units, municipal zoning ordinances must comply with the following conditions.” Within those conditions, a town can set a maximum ADU size and apply the same setback and dimensional standards it applies to the single-family dwelling.
Two carve-outs matter for siting. First, an ADU is not exempt from shoreland zoning, but a town cannot ban it outright there: under subsection 5, “a municipality may not categorically prohibit accessory dwelling units in the shoreland zone.” Second, both the ADU rule and the density rule open with a subsurface wastewater exception. Section 4364-B, subsection 1, applies “Except as provided in Title 12, chapter 423-A,” which is Maine’s statewide plumbing and subsurface wastewater program.
Septic and water must be verified. Under subsection 7-B, an applicant on septic must show “proof of adequate sewage disposal for subsurface wastewater,” and “The septic system must be verified as adequate by a local plumbing inspector.” Plans “for subsurface wastewater disposal must be prepared by a licensed site evaluator.” A narrow geographic exception applies in the Lewiston and Auburn drinking-water watershed: under subsection 1-B, the rule “does not apply to a lot … within the watershed of a water source … in the City of Lewiston or the City of Auburn.”
One point is unsettled. The statute frames a town’s ADU duties around its “municipal zoning ordinances,” and whether a town with no zoning ordinance at all is effectively exempt is not squarely addressed in the statutory text. As of October 18, 2026, the statute text still does not address unzoned towns directly, so treat that question as open and confirm it locally.
When did Maine’s ADU law take effect?
The law itself took effect in 2022, but the dates that matter for enforcement are the municipal compliance dates, and they are not the widely cited July 1, 2023 figure. July 1, 2023 was the original compliance deadline in LD 2003 as first enacted, and it was superseded by later amendment.
The current statutory implementation date was reset by LD 1706, Public Law 2023, chapter 192, and it is now tiered. Under 30-A M.R.S. section 4364, subsection 1-A, the implementation date is “January 1, 2024 for municipalities for which ordinances may be enacted by the municipal officers without further action or approval by the voters,” and “July 1, 2024 for all other municipalities,” which are the town-meeting towns. Section 4364-B adopts that same date: its subsection 1-A says the “implementation date” “has the same meaning as in section 4364, subsection 1-A,” and subsection 13 confirms “A municipality is not required to implement the requirements of this section until the implementation date.”
The Maine Department of Economic and Community Development, through its Housing Opportunity Program, confirms the first deadline as “January 1, 2024, for towns that do not utilize town meeting.” So the compliance dates now in force are January 1, 2024 for council or officer towns and July 1, 2024 for town-meeting towns.
How does Maine’s ADU rule connect to its density law?
Maine’s ADU mandate sits next to a broader density rule, 30-A M.R.S. section 4364-A, titled “Residential areas, generally; up to 4 dwelling units allowed.” That section, not the ADU section, is what allows multiple units on a single lot. Under one version of the text, a municipality “shall allow at a minimum … Three dwelling units, attached or detached, including accessory dwelling units, per lot,” and “Four dwelling units … per lot if the lot is located in a designated growth area” or served by public water and sewer.
One technical caveat, and it matters less than it looks. The Maine Revisor prints two versions of section 4364-A, subsection 1, flagged as in conflict: one from Public Law 2025, chapter 655 and one from Public Law 2025, chapter 733. But both versions set the same floor: at least three dwelling units per lot, counting ADUs, and at least four where the lot is in a designated growth area or on public water and public sewer. The two versions word the growth-area and water-system tests slightly differently, so if your lot sits on that line, confirm with your town. The single-ADU right in section 4364-B is separate and is not affected.
How Maine compares to nearby states
Maine’s statewide by-right ADU mandate resembles reforms in several other states, each covered on its own page. See the New Hampshire ADU law page for a neighboring state’s approach, the Massachusetts ADU law page for the region’s by-right law, and our tiny house in a backyard guide for how small-footprint units fit under ADU rules. For the full set of state pages, visit the ADU laws library.
What changed in 2026?
Maine’s ADU statute changed again in the 2026 session. The Revisor’s current text of sections 4364, 4364-A and 4364-B carries amendments from Public Law 2025, chapters 648, 655 and 733, none of which were in the text as of October 1, 2025. The ones that matter for a homeowner:
- The guaranteed ADU now extends to lots with a 2- or 3-unit building, not just a single-family home (section 4364-B, subsection 3, paragraph A).
- The planning board bar moved from section 4364-B to section 4364-A, subsection 5-A, and still covers ADUs.
- Towns may now exempt lots in coastal barriers, special flood hazard areas and coastal sand dune systems (section 4364-B, subsection 8, paragraph D).
- Section 4364-A’s caps on minimum lot size and lot area per unit were rewritten (subsection 2-A). On a growth-area lot with public water and sewer, a town may not require more than 5,000 square feet of lot or more than 1,250 square feet of lot area per unit for the first four units.
The January 1, 2024 and July 1, 2024 implementation dates in section 4364 did not change.
What changes in 2027?
No further statewide Maine ADU changes are scheduled as of October 18, 2026. The section 4364-A conflict between chapters 655 and 733 is a drafting overlap the Revisor will reconcile; both versions carry the same three- and four-unit floor. We recheck the Maine Legislature and Revisor each session and will update this page when new text is enacted or the conflict is resolved.
Frequently Asked Questions
Does Maine allow ADUs?
Yes. Maine requires every municipality to allow at least one accessory dwelling unit on any lot whose principal structure is a single-family home or a 2- or 3-unit building, under 30-A M.R.S. section 4364-B.
What is LD 2003?
LD 2003 is Maine’s 2022 housing law, codified as Public Law 2021, chapter 672. It created the statutes that mandate ADUs and higher residential density, including 30-A M.R.S. section 4364-B for accessory dwelling units.
How many ADUs can I build in Maine?
At least one ADU is guaranteed under 30-A M.R.S. section 4364-B. Separately, section 4364-A guarantees at least three dwelling units per lot, counting ADUs, and four on a lot in a designated growth area or on public water and sewer, so the real ceiling is usually the unit count, not an ADU cap.
When did Maine’s ADU law take effect?
The law took effect in 2022, but the municipal compliance dates now in force are January 1, 2024 for towns whose officers can enact ordinances and July 1, 2024 for town-meeting towns. These superseded the original July 1, 2023 deadline.
Can my town require owner occupancy or extra parking for an ADU?
No. A Maine municipality cannot require the owner to live on the lot, under 30-A M.R.S. section 4364-B, subsection 4-E, and cannot impose parking beyond what the single-family dwelling requires, under subsection 4-C.
How big can a Maine ADU be?
A Maine ADU must be at least 190 square feet, under 30-A M.R.S. section 4364-B, subsection 6. A municipality may set a maximum size, but it cannot set a minimum higher than 190 square feet.
Can I build an ADU in the shoreland zone?
Yes, but shoreland zoning standards still apply. Under 30-A M.R.S. section 4364-B, subsection 5, a municipality may not categorically prohibit accessory dwelling units in the shoreland zone.
What about septic and water for an ADU?
An ADU on septic needs adequate sewage disposal verified as adequate by a local plumbing inspector, under 30-A M.R.S. section 4364-B, subsection 7-B. Subsurface wastewater plans must be prepared by a licensed site evaluator.
Sources
- Maine Revisor of Statutes, 30-A M.R.S. section 4364-B (Accessory dwelling units): https://legislature.maine.gov/statutes/30-A/title30-Asec4364-B.html
- Maine Revisor of Statutes, 30-A M.R.S. section 4364 (Affordable housing density and implementation date): https://legislature.maine.gov/statutes/30-A/title30-Asec4364.html
- Maine Revisor of Statutes, 30-A M.R.S. section 4364-A (Residential areas, up to 4 units): https://legislature.maine.gov/statutes/30-A/title30-Asec4364-A-2.html
- Maine Department of Economic and Community Development, Housing Opportunity Program update (implementation deadline): https://www.legislature.maine.gov/doc/10379
Disclaimer
This page is general information, not legal advice. ADU law changes often and towns apply it differently. Confirm the current rules with your local planning department before you build.