ADU rules in Portland

A detached backyard cottage is allowed by right. Sec. 6.6.2(A)(1) permits ADUs on all lawfully conforming and nonconforming lots with legal residential uses, and Sec. 6.6.2(A)(2) expressly allows them as new detached accessory structures. Portland goes past the state floor by allowing up to two ADUs per qualifying property (Sec. 6.6.2(A)(3)), and ADUs are exempt from maximum lot coverage and minimum lot area per dwelling unit. A detached ADU is capped at 18 feet, or 25 feet if built above a garage, takes the small setbacks of a detached accessory structure under 250 square feet (5 feet at the rear and 3 to 5 feet at the interior side in the RN zones), and may not be placed between the principal structure and the front lot line.

Interior, basement and addition conversions are on exactly the same footing as detached units. Sec. 6.6.2(A)(2) allows an ADU as a building addition or within an existing lawfully conforming or nonconforming structure, and the Article 3 definition treats an accessory dwelling unit as one subordinate in size to the principal residential structure and located either within it or in an accessory structure. The city's Zoning FAQ confirms an ADU may be created within the principal building, within a legally existing detached building, or in a new building or addition, and that converting an attached or detached garage can qualify. The one real limit is that adding an ADU may not increase the degree of nonconformity of any structure.

Portland sets no square-foot ceiling. Sec. 6.6.2(A)(5) ties each ADU to two thirds of the gross floor area of the largest principal unit on the lot, so a 1,800 square foot house supports an ADU of up to 1,200 square feet while a small cottage supports much less. Where two ADUs are built, the aggregate square footage of the detached ones may not exceed the gross floor area of the principal structure. Height rather than floor area is usually the binding constraint on a detached unit at 18 feet, or 25 feet over a garage. ADUs are exempt from maximum lot coverage and from minimum lot area per dwelling unit (Sec. 6.6.2(A)(6)), and the statewide floor is a 190 square foot minimum under 30-A M.R.S. sec. 4364-B(6).

No. The phrase 'owner occupied' does not appear anywhere in the 385 page Land Use Code, and Sec. 6.6.2(A) conditions an ADU only on the lot having a legal residential use, never on who lives there. The one place owner occupancy matters is short-term rental licensing, and there Chapter 6 cuts the other way: Sec. 6-150.1 states that accessory dwelling units are not considered owner-occupied units for short term rental registration and regulation. Maine will bar municipal owner-occupancy requirements outright under 30-A M.R.S. sec. 4364-B(4)(E), but that provision does not reach any municipality until July 1, 2027 (PL 2025, c. 385, sec. 23 as amended by PL 2025, c. 733, sec. 25), so Portland's answer today rests on its own ordinance rather than on preemption.

Sec. 18.1.2 is one sentence long: off-street vehicle parking is not required. Article 18 now sets parking maximums rather than minimums, and Sec. 18.1.4(D) adds that off-street parking is not calculated separately for accessory uses, so an ADU adds nothing to the parking math even where an owner chooses to build spaces. A Transportation Demand Management plan is required for certain larger development actions under Sec. 13.6.1, which a single ADU will not trigger. The state ceiling in 30-A M.R.S. sec. 4364-B(4)(C), which forbids any parking requirement beyond that of the single-family dwelling on the lot, is therefore moot in Portland.

Administrative. An ADU is an accessory use permitted by right wherever residential uses are legal (Sec. 6.6.1(A) and Sec. 6.6.2(A)(1)); there is no conditional use hearing before the Zoning Board of Appeals and no Planning Board approval. What a homeowner needs is a building permit from the Building Authority, which is the Department of Permitting and Inspections, plus a certificate of occupancy for the change of use under Sec. 6.3; the city's Zoning FAQ says plainly that creation of an ADU requires a building permit. Bigger projects can pick up a minor site plan review, decided by the Planning Authority at staff level, once new construction on a one to four family lot reaches 1,000 square feet or disturbs 1,000 square feet of ground (Table 13-A and Table 13-B). In a local historic district, historic preservation approval under Sec. 16.5.1 is a condition precedent to the building permit.

Any ADU that is rented is a rental unit that must be registered with Permitting and Inspections annually by December 31, at $55 per unit per year for a long-term registration as of July 1, 2026. The big win is Sec. 6-231(f): accessory dwelling units are exempt from Portland's rent control and tenant protections ordinance, so the citywide allowable increase percentage does not bind an ADU's rent. Short-term renting is possible but tightly managed: Sec. 6-150.1 means an ADU is always counted as non-owner occupied even when the owner lives in the front house, and non-owner-occupied mainland short-term rentals are capped at 1.5 percent of the prior year's registered long-term rentals, which the city set at 293 units for 2026. Sec. 6-153(b) carves newly built ADUs out of that cap for five years, and the city's short-term rental page spells it out: a mainland ADU can be licensed for up to five years from the date of the certificate of occupancy, after which it goes on the non-owner-occupied waitlist or becomes a registered long-term rental. Non-owner-occupied registration fees start at $200 for a first unit and escalate, and no person or entity may hold more than five short-term rental registrations citywide.