The Maine Agenda › Live in Maine

ADU Rules by Town in Greater Portland, Maine

Published August 8, 2026 · 11 min read

The most useful thing to know about building an accessory dwelling unit in Greater Portland is that the question most people ask about it is now largely obsolete.

For years the advice was to start with your town. Look up your zoning district, find out whether your town allows a backyard cottage, check whether it will make you live on the property. That advice built an entire genre of web pages, one per town, most of them written by companies that sell prefabricated cottages. Those pages are still there, and a good number of them are now wrong, because the state of Maine spent four years quietly removing the things they describe.

As of July 1, 2026, in every Greater Portland municipality whose council can pass an ordinance on its own, state law sets a floor your town cannot go under. It is a generous floor. What is left for towns to decide is a much shorter list than the internet suggests, and knowing which list a rule belongs to is the whole game.

Everything below comes from the statute itself, from the Maine Office of Community Affairs guidance issued in March 2026, and from each town's own ordinance or published page. Where we could not read a town's ordinance this week, this guide says so instead of guessing.

The floor: nine things your town can no longer do

Maine's ADU statute is 30-A M.R.S. section 4364-B. It started with LD 2003 in 2022, was amended in 2023, and was amended again by LD 1829, enacted as Public Law 2025, chapter 385. The 2025 amendments are the ones that matter now.

Your town may not prohibit at least one accessory dwelling unit on any lot where a single-family dwelling is the principal structure, anywhere residential uses are permitted, including where they are permitted as a conditional use. The 2025 amendment extended that to lots with multi-unit structures as well.

Your town may not count that one ADU against density requirements or calculations for the district.

Your town may not require additional parking for it. Not one space. This applies inside and outside designated growth areas, which is unusual: the parking rules for other kinds of housing in Maine are geographically limited, and this one is not.

Your town may not require planning board approval. The state guidance is explicit that "planning board" means any appointed or elected board or committee, and equally explicit that code enforcement review, staff review and site plan review are still allowed. The distinction is between a public board that can say no and staff who check whether you met the standards.

Your town may not require you to live on the property. This is the biggest single change, it is new in the 2025 amendments, and it is the rule most likely to still be stated the old way on a page you find. The state guidance addresses it twice, including a direct question about whether owner-occupancy can be used to discourage short-term rentals. The answer given is no.

Your town may not prohibit an ADU on a lot that does not conform to current zoning, as long as the ADU does not make the nonconformity worse. It also may not refuse an ADU that was built without municipal approval, if that unit otherwise meets state and local ADU requirements.

Your town may not categorically ban ADUs in the shoreland zone. Shoreland standards still apply in full. A blanket prohibition does not.

Your town may not require fire suppression sprinklers unless the ADU sits within or attached to a structure containing more than two dwelling units, counting the ADU itself. A detached cottage behind a single-family house does not trigger a municipal sprinkler mandate.

Your town may not count an ADU permit against a rate of growth ordinance. Falmouth, for example, caps development permits at 58 a year town-wide for single-family, two-family and manufactured housing units combined. An ADU does not consume one of those.

The number nearly every ADU page gets wrong

The statute says an ADU must be at least 190 square feet, and then says that if the Technical Building Codes and Standards Board adopts a different minimum, the Board's number governs.

The Board adopted a different number. According to the March 2026 state guidance, the minimum size has been 160 square feet since the Board set it in rule in March 2025.

So 190 is the number printed in the statute and 160 is the number that actually applies. Yarmouth saw this coming and drafted around it: rather than write a figure into its ordinance, Yarmouth's standard simply points at whatever the Board has adopted. That is the correct way to write a local rule on top of a state rule, and it is worth noticing because most towns did the opposite.

What your town still controls

Four things, plus money.

Maximum size. The state sets a floor and no ceiling. Towns set the ceiling, and this is where the real variation lives.

Short-term rentals. The state guidance is direct: a town cannot use owner-occupancy to stop ADUs becoming short-term rentals, but it can regulate short-term rentals as such. Whether your ADU can go on Airbnb is a question for your town's short-term rental ordinance, not its ADU ordinance, and those diverge sharply across the region. Our short-term rental rules by town guide covers which six towns run a registration program and what each charges.

Process and review. Site plan review, historic district review and code enforcement review all survive. So does the written verification that your water and wastewater can carry a second household, which is a real constraint on septic lots and not a formality.

Design standards. Towns can still regulate entrances, exterior appearance and setbacks, as long as their setback rules for ADUs are no stricter than for the principal dwelling.

And fees. Nothing in state law caps what a town charges, which is why the cost of the same project varies by a factor of three across a fifteen minute drive.

The four towns we verified this week

Portland is the most permissive in the region and has been since December 2020, when the City Council adopted a provision allowing up to two accessory dwelling units on any residential property. Two, not one. State law guarantees one and expressly permits towns to be more generous. The standards sit in Section 6.6.2.A of Chapter 14 of the Land Use Code. An ADU can go inside the principal building, inside a legally existing detached building, or in new construction, and the route is a building permit rather than a board. Detached ADUs are capped at 18 feet in height, or 25 feet above a garage, and cannot sit between the principal structure and the front lot line. New ADUs pay the city's impact fees.

One caution on Portland: the city's own zoning FAQ still tells homeowners that creating an ADU "may require installation of a new sewer line and installation of a fire sprinkler system." The sewer half is real. The sprinkler half is now narrower than that sentence suggests, because state law bars a municipal sprinkler requirement for an ADU unless the structure holds more than two units. If you are quoted a sprinkler requirement for a cottage behind a single-family house, ask which authority it comes from. Zoning staff are at 207-874-8709.

South Portland rewrote its rules to make ADUs larger and to drop the requirement that they be attached to the house. Standards are in Chapter 27, Sections 27-1576 through 27-1579. The process is the most structured in the region and the most expensive to start: a zoning permit costs 500 dollars total, split into a free pre-application meeting, a 200 dollar application meeting, and a 300 dollar submittal fee, and that is before the building permit. Budget three to four weeks for zoning review and four to six for the building permit, which can run simultaneously. Most applications involve no public hearing. Code Enforcement is at 207-767-7603.

Yarmouth has had ADUs since 2006 and had roughly 50 to 60 of them as of its own May 2024 guide, which is the most thorough municipal ADU document in the region. The cap is 900 square feet, rising to 1,215 square feet if you are converting part of an existing structure such as a barn. Review is administrative, as a Minor Site Plan under Chapter 702, Article I.J.13, and costs 150 dollars plus 5 dollars for each abutter notification letter within 500 feet. Expect six to eight weeks. Before you get a building permit you must record a declaration of restrictions at the registry of deeds stating that the unit will not be sold separately. There is one more Yarmouth quirk worth knowing: the town does not regulate rentals shorter than 30 days in any dwelling unit, so an ADU there can be a short-term rental.

Two caveats on Yarmouth. Its guide is dated May 2024 and therefore predates the 2025 amendments. And the guide states that the Planning Director may refer an application to the Planning Board for a public hearing, which sits uneasily beside the state guidance that planning board approval may not be required. If that comes up, it is a question worth asking in writing.

Falmouth lists accessory dwelling units as a permitted use, not a conditional use, in its Route One North, Mixed Use Cluster and Village Mixed Use districts, and its parking section says plainly that the permitting authority "may not impose any parking requirements for accessory dwelling units." That language went in on December 11, 2023, ahead of the state deadline. The version we read is Ordinance 105-2025, attested by the town clerk on May 28, 2025. We did not locate Falmouth's current maximum ADU size in that document, and the figures circulating on commercial ADU sites for Falmouth include an owner-occupancy requirement that state law no longer permits, so we are not repeating any of them here.

What we could not check this week

We did not read the current ADU provisions for Cape Elizabeth, Scarborough, Freeport, Cumberland, Gorham, Westbrook or Windham. That is a statement about our week, not about those towns. Each of them is subject to the same state floor described above, and each of them may have a maximum size, a fee and a process we have not seen. Call the code enforcement office and ask for the ADU standards by section number.

Two dates, and which one applies to you

LD 1829 took effect on September 24, 2025, but municipalities were given time to conform. The state's implementation deadline is July 1, 2026 for municipalities whose council can enact ordinances on its own, and July 1, 2027 for town meeting municipalities.

Portland, South Portland, Yarmouth and Falmouth all enact zoning by council vote, which we confirmed from the ordinances and council actions themselves. For those four, the deadline has passed. If your town still runs zoning changes through town meeting, its deadline is next summer, and an owner-occupancy clause on the books today may still be enforceable there until then. Ask your code officer which category your town is in before assuming either way.

One more thing on the horizon: a bill numbered LD 2173 has been proposed to clarify parts of LD 1829, including replacing the phrase "multi-unit structure" with "two-unit or three-unit residential structure." It had not been enacted as of the March 2026 state guidance. Do not plan a project around it.

A warning about searching for this

Both of Maine's classic search traps are unusually bad on this topic.

Search for Portland ADU rules and the top result is likely to be portland.gov, which is Portland, Oregon, whose ADU code is Chapter 33.205. Portland, Maine is portlandmaine.gov and its code is Chapter 14. Search for Falmouth ADU rules and you will get falmouthma.gov and its Zoning Bylaw Section 240-9.1, which is Falmouth, Massachusetts. Our Falmouth is falmouthme.org.

The second trap is subtler and more expensive. The commercial ADU pages that rank well for these queries are templated, and several of them describe Maine towns as still requiring owner occupancy for seven months of the year. That was true once. It is not the law now. A page can be accurate about setbacks and wrong about the thing that decides whether your project works.

If you are weighing an ADU as an income property rather than a place for a parent, the arithmetic runs through property taxes by town and, if you plan to rent long term, through rent increase rules by town.

FAQ

Can I build an ADU in Greater Portland without my town's permission?

No, but you no longer need a board's permission. Maine law requires every municipality to allow at least one accessory dwelling unit on a lot with a single-family dwelling wherever residential uses are permitted, and prohibits requiring planning board approval. You still need a building permit, and your town can still run staff-level code enforcement, site plan and historic review.

Do I have to live on the property to have an ADU in Maine?

No. Public Law 2025, chapter 385 added a provision requiring municipalities to allow construction and occupancy of an ADU even when the lot owner does not live in any dwelling on that lot. The state guidance confirms a town may not require the owner to live in either the ADU or the primary structure, including as a way to discourage short-term rentals.

What is the minimum size for an ADU in Maine?

160 square feet. The statute names 190 square feet but defers to the Technical Building Codes and Standards Board, and according to the state's March 2026 guidance the Board set the minimum at 160 square feet in rule in March 2025. Towns may set a maximum size but not a higher minimum.

How much does an ADU permit cost near Portland, Maine?

It varies by more than three times across towns we verified. Yarmouth charges 150 dollars for administrative review plus 5 dollars per abutter letter within 500 feet. South Portland charges 500 dollars for the zoning permit alone, split across two payments. Both are separate from the building permit, and Portland additionally charges city impact fees on new ADUs.

Does my town have to give me extra parking relief for an ADU?

Your town cannot require any additional parking for an ADU, anywhere in Maine, inside or outside a growth area. This is stronger than Maine's general housing parking rules, which are limited to designated growth areas.

Can I rent my ADU on Airbnb?

That depends entirely on your town's short-term rental ordinance, which is a separate body of rules. Yarmouth does not regulate rentals under 30 days at all. Portland, South Portland, Falmouth, Cape Elizabeth, Freeport and Scarborough all run registration programs with their own caps and fees, covered in our short-term rental guide.

Will my town make me install fire sprinklers in an ADU?

Not by ordinance, unless the ADU is inside or attached to a structure with more than two dwelling units counting the ADU. That limit came in with the 2025 amendments. Separate local fire prevention ordinances tied to total building area can still apply to large additions, so ask which ordinance a sprinkler requirement is being cited under.