Maine ADU Law: Permits, Property Taxes, and Rentals

Maine’s ADU law requires every municipality in the state to allow at least one accessory dwelling unit on any lot with a single-family home as the principal structure. The rule lives at Title 30-A, §4364-B, grew out of LD 2003, and has been amended as recently as 2025.¹ You can build the ADU inside your existing home, attached to it, or as a separate structure on the lot. The state sets the floor; your town fills in the details; and before any of that matters, you have to show the property can handle the water and wastewater load.

What the State Guarantees and What Towns Still Control

The statute strips away several of the restrictions towns used to impose. A municipality cannot prohibit ADUs outright. It cannot count your ADU toward a rate-of-growth cap. It cannot impose density requirements on a single ADU per lot, cannot require extra parking beyond what your single-family home already needs, and cannot apply stricter setbacks to an attached or interior ADU than it applies to the main house.¹

A 2025 amendment removed owner-occupancy requirements. Towns must now allow ADU construction and occupancy even when the property owner does not live on the lot.² If your town’s ordinance still requires you to live in the main house or the ADU, that provision is no longer enforceable.

ADUs are also allowed on nonconforming lots, provided the new unit does not increase the existing nonconformity. And an ADU built without municipal approval in the past can be legalized going forward as long as it meets current requirements.¹ That matters for owners who finished a basement apartment years ago without pulling permits.

What towns can still do: regulate size, height, and design standards; run site plan review if local ordinance authorizes it; and require conditional-use approval in zones where residential use is conditional rather than permitted by right.³ The state sets a minimum ADU size of 190 square feet (unless the Technical Building Codes and Standards Board adopts a different figure) but no maximum. Maximum sizes vary widely by town, so read the local zoning ordinance before you commit to a floor plan.¹

Shoreland zoning is a separate layer. Towns cannot prohibit ADUs in the shoreland zone, but shoreland standards can still restrict where and how one is built. If your lot sits within 250 feet of a river, lake, or tidal area, expect added setback and impervious-surface rules.

Water and Wastewater Verification

Before a municipality can issue a certificate of occupancy, you have to provide written verification that the ADU has adequate water and wastewater service. The required proof depends on your setup:

On a septic system: proof of adequate subsurface wastewater disposal verified by a local plumbing inspector, with the disposal plan prepared by a licensed site evaluator.
On public water or sewer: proof of adequate service for the additional flow, plus proof of payment for the connection.
On a private well: test results showing the water is potable and suitable for domestic use.

Once the local plumbing inspector provides written verification, the municipality cannot demand additional water or wastewater documentation before issuing occupancy.²

For properties on older septic systems, this step is often the most expensive part of the project. A system sized for a three-bedroom house decades ago may not have capacity for a second dwelling. Upgrading or replacing a septic system in Maine can easily add tens of thousands of dollars to the budget. Get a site evaluation early, before you pay for design work.

Permits and the Local Process

State law lets municipalities set their own permitting process for ADUs as long as it stays consistent with §4364-B.³ The law does not require streamlined permitting or set timelines, so what you face depends on your town.

Expect to submit plans showing footprint, height, setbacks, and compliance with local dimensional standards. If your town uses site plan review for residential development, your ADU may go through it. Expect to document water and wastewater connections. In some communities you will file separate plumbing, electrical, and building permits.

Fees vary. Some towns charge a flat application fee, others base it on estimated construction value. Impact fees are possible but less common for a single ADU in Maine. Start at your town’s code enforcement office; they can give you an application checklist and tell you which local officials (planning board, code enforcement officer, plumbing inspector) you need to work with.

What an ADU Does to Your Property Taxes

Adding an ADU raises your assessed value and your tax bill. Maine’s constitution requires real estate to be assessed at its just (market) value, and municipal assessors update records when new construction occurs.⁴ The assessor evaluates the improvement based on size, construction quality, and the effect on overall market value.

The actual increase depends on how much value the assessor adds and your town’s mil rate. A $120,000 ADU in a town with a 15 mil rate adds roughly $1,800 per year in property tax. The bump typically appears on the next tax bill after your ADU receives its occupancy certificate.

Maine’s homestead exemption reduces the taxable value of your primary residence by up to $25,000.⁵ Building an ADU does not disqualify you from the exemption, but the exemption applies to your homestead, not to a rental unit. The value attributable to the ADU itself is fully taxable at the local rate.

Federal Taxes if You Rent It Out

Rental income from an ADU is treated like any other residential rental. You report it on Schedule E (Form 1040), listing gross rents received and subtracting allowable expenses.⁶ Deductible expenses include the rental portion of mortgage interest, property taxes, and insurance, along with repairs, cleaning, utilities you cover for tenants, advertising, and property management fees. You can also depreciate the ADU structure over 27.5 years using the straight-line method.⁷

If the ADU is inside your primary home rather than detached, shared expenses have to be divided between the rental portion and the personal portion by a reasonable method such as square footage. Only the rental share goes on Schedule E.⁷

The 14-Day Rule

If you rent the ADU for fewer than 15 days during the tax year, you do not have to report the income. The tradeoff: you also cannot deduct any rental expenses for that period.⁸

If you personally use the ADU for more than 14 days or 10% of total rental days (whichever is greater), the IRS classifies it as a personal residence, which caps your rental expense deductions at the amount of gross rental income.⁸

Insurance Changes to Make

Your existing homeowners policy probably was not written with a second dwelling in mind. An attached or interior ADU is generally covered as part of the primary dwelling under a standard policy, at least when the occupants are family rather than paying tenants. A detached ADU falls under the “other structures” portion of most policies, which is often capped at 10% of total dwelling coverage. If your home is insured for $300,000, that leaves only $30,000 for the detached structure, which may not approach rebuilding cost.

Renting the ADU, long-term or through a platform like Airbnb, changes things again. Most standard homeowners policies exclude or limit coverage for rental activities. Plan on landlord or rental property insurance, which covers damage to the unit, liability to tenants and guests, and loss of rental income during covered repairs. For short-term rentals, some insurers offer a home-sharing endorsement; others require a separate landlord or business policy. An umbrella policy adds liability coverage above the underlying limits and is worth considering once tenants or paying guests are in the picture.

Private Restrictions the State Law Does Not Override

Section 4364-B explicitly leaves private agreements alone. It does not interfere with the validity of any easement, covenant, deed restriction, or other agreement between private parties that imposes greater restrictions than the law provides.²

If your deed limits the property to a single dwelling or your homeowners association bans accessory structures, those restrictions remain enforceable even though the state and your town would otherwise allow the ADU. A title search or a short consultation with a real estate attorney can surface these problems before you spend money on design.

Short-Term Rentals

State law does not address whether an ADU can be used as a short-term rental. That is a local question. Many Maine towns impose minimum rental periods (often 30 days), registration or licensing requirements, and caps on the number of short-term rentals in a given area, with coastal and tourism-heavy communities regulating most tightly. Confirm with your local planning department or code enforcement office before listing an ADU anywhere.

Financing an ADU

Fannie Mae now treats ADUs as a standard home feature and allows ADU financing through any of its standard loan products. HomeStyle Renovation lets you finance ADU construction as part of a purchase or refinance mortgage. Construction-to-Permanent covers simultaneous construction of a new home and an ADU on the same lot. HomeReady lets buyers or refinancers of a home with an existing ADU count the ADU’s rental income toward loan qualification.

The Fannie Mae rules require that the ADU include independent living, sleeping, cooking, and bathroom space, and that it be accessible without passing through the primary residence. Properties with multiple ADUs, 2-to-4-unit properties, and manufactured homes serving as the primary residence are ineligible.⁹

Home equity loans and HELOCs are another route for owners with substantial equity. They usually carry higher rates than a first mortgage but let you keep a favorable existing rate in place.

1
Maine State Legislature. Maine Revised Statutes Title 30-A 4364-B – Accessory Dwelling Units
2
Maine State Legislature. Maine Revised Statutes Title 30-A 4364-B – Accessory Dwelling Units
3
Maine Department of Economic and Community Development. LD 2003 Guidance
4
Maine Revenue Services. Understanding the Process and the Myths of Revaluations
5
Maine Revenue Services. Homestead Exemption Program FAQ
6
Internal Revenue Service. Instructions for Schedule E (Form 1040)
7
Internal Revenue Service. Publication 527 – Residential Rental Property
8
Internal Revenue Service. Topic No. 415 – Renting Residential and Vacation Property
9
Fannie Mae. Accessory Dwelling Units