ADU rules in Biddeford

Detached backyard ADUs are expressly authorized. Land Development Regulations Art. VI, Section 78.C.1 lets the Code Enforcement Office issue a building permit for an ADU 'in a detached structure on a single-family home lot,' and Art. V, Table A lists accessory dwelling units as a permitted (P) use in exactly the 11 zones where a single-family dwelling is permitted: SR-1, CR, R-1-A, R-1-B, R-2, R-3, W-1, W-3, R-F, OR and MSRD-2. Table A note 27 adds nonresidentially zoned lots that held a nonconforming single-family dwelling as of January 1, 2017. One ADU per parcel, and none in the RP (Resource Protection) shoreland overlay. Detached units carry extra design conditions: at least 10% of any facade facing an alley or public street must be windows, and exterior stairway railings must match the unit's finish or trim (no raw lumber).

The same permit provision covers a unit created 'within an existing or new ... single-family dwelling' (an internal conversion such as a basement or upstairs unit) and one 'attached to' the dwelling, so internal, attached and detached ADUs all run through one administrative Code Enforcement review. Internal and attached units may not add an entrance facing a public street on the primary structure, and any stairway serving them must be enclosed. An attached unit's primary exterior materials must match the principal structure's. Only one ADU may be created within a single-family house or on a house lot.

Section 78.C.1.f caps an ADU at 35% of the principal dwelling's gross floor area or 900 square feet, 'whichever is greater,' so on larger houses 900 sq ft is a floor rather than a ceiling: a 3,000 sq ft home supports a 1,050 sq ft ADU. Section 78.C.1.g makes that permanent by barring any later enlargement past the same limit. The minimum is 300 square feet (Sec. 78.C.1.e) and no ADU may have more than two bedrooms (Sec. 78.C.1.j). Rooftop decks are prohibited and balconies may not face interior side lot lines. Biddeford sets no separate ADU height limit; a detached unit takes the ordinary accessory-structure and district dimensional standards. Note that the local 300 sq ft minimum sits above the 190 sq ft minimum in 30-A M.R.S. Sec. 4364-B(6), which lets a municipality impose a maximum size but says nothing about raising the minimum.

Biddeford requires it and that requirement is currently lawful. Section 78.D.1 provides that the owner of the residence in which the ADU is created 'must continue to occupy either the ADU or the principal dwelling unit,' and Sec. 78.D.2 orders the owner to remove the features that make the ADU a dwelling unit if they cannot. Maine has legislated a ban on municipal owner-occupancy requirements, 30-A M.R.S. Sec. 4364-B(4)(E), but it is future law: PL 2025, c. 385, Sec. 17 enacted it and Sec. 23 of that chapter set its applicability, and PL 2025, c. 733, Sec. 25 (LD 2173, signed April 16, 2026) then struck the earlier July 1, 2026 branch of that applicability clause so it reads 'this Act applies to municipalities beginning July 1, 2027,' with c. 733, Sec. 26 making the change retroactive to September 24, 2025. Every Maine municipality, Biddeford included, therefore keeps its owner-occupancy rule until July 1, 2027. Plan on living in the house or the ADU, and re-check this field after that date.

Section 78.C.1.l requires one off-street parking space for an ADU 'unless legal on-street parking or publicly supplied off-street parking is located within 1,000 feet,' and allows tandem parking. That is an ADU-specific space on top of the two spaces per dwelling unit that Table 6-49B requires for residential uses, which is what 30-A M.R.S. Sec. 4364-B(4)(C) forbids: an ADU 'may not be subject to any additional parking requirements beyond the parking requirements of the single-family dwelling unit on the lot where the accessory dwelling unit is located.' Unlike the owner-occupancy ban, this paragraph is fully operative today. It was enacted by LD 2003 (PL 2021, c. 672, Sec. 6) as part of the original section, neither PL 2025, c. 385 nor PL 2025, c. 733 touched subsection 4, paragraph C, and it has bound Biddeford since the January 1, 2024 implementation date that 30-A M.R.S. Sec. 4364(1-A)(A) sets for council-enacted municipalities. In practice the ordinance's own 1,000-foot exemption already covers most in-town lots.

Purely administrative. Section 78.B.1.a provides that 'a permit for an ADU may be issued by the Code Enforcement Office so long as all of the standards in this section are met,' and Sec. 78.B.1.b puts the review in that office, whose findings are final except on appeal to the Zoning Board of Appeals under Art. IX. There is no Planning Board review, public hearing or conditional-use permit, and Art. V, Table A marks ADUs 'P' (permitted use) rather than 'C' (conditional use) in every zone where they appear. Budget a $200 nonrefundable ADU application fee on top of the building, electrical and plumbing permit fees (Sec. 78.B.2.a). No ADU may be occupied until Code Enforcement issues a certificate of occupancy (Sec. 78.E.2), and 30-A M.R.S. Sec. 4364-B(7) requires written verification of adequate water and wastewater service before that certificate issues.

Renting the unit is the point: Sec. 78.A.1 and .A.2 describe ADUs as a way for homeowners to obtain 'rental income' and to 'add moderately priced rental units to the housing stock.' The practical limit is that Sec. 78.D.1 requires the owner to keep occupying the ADU or the principal dwelling, so only one of the two units can be let until 30-A M.R.S. Sec. 4364-B(4)(E) reaches municipalities on July 1, 2027. The unit cannot be split off and sold: Sec. 78.C.1.d says ADUs 'shall not be intended for sale and shall remain in common ownership with the principal dwelling unit.' Section 78.D.3 caps occupancy at three people and Sec. 78.C.1.j at two bedrooms. Biddeford imposes no short-term-rental registration, licensing or cap: full-text searches of the Code of Ordinances and Land Development Regulations for 'short-term rental' and 'transient' return only unrelated definitions plus the separate bed-and-breakfast conditional use at Art. VI, Sec. 9, and Maine has no statewide STR regime. The city's multifamily rental registration and life-safety inspection program does not reach a house plus an ADU either, because Code Sec. 18-27 defines a multifamily dwelling as a building containing three or more dwelling units.