ADU rules in South Portland

Sec. 27-1578(a)(iii) expressly allows an ADU created by "Incorporating an ADU into a new structure or constructing a detached ADU structure", and the use tables list "Accessory Dwelling Units (ADUs)*" as a P (permitted) use in every one of the eight residential districts (RF, AA, A, RC, G, VR, RT, WR) and in the mixed-use, commercial, industrial and conditional districts wherever dwellings are allowed. There is no S (special exception) entry for ADUs anywhere in Chapter 27. A detached unit carries its own dimensional rules rather than a discretionary gate: exterior mechanical equipment must stay out of the setback and at least 10 feet from a property boundary, transparent openings must be at least 5 feet from the boundary, the ADU's primary entrance must be at least 10 feet from a boundary unless it is an existing opening on a structure that existed on September 1, 2022, and a detached ADU may not exceed 28 feet in height. Converting a lawfully existing nonconforming detached structure is expressly allowed, including raising its roof to meet building code, but not if the ADU would sit within 12 feet of an inhabited structure on an adjacent lot.

Sec. 27-1578(a)(i) and (a)(ii) put internal conversions and attached additions on exactly the same footing as detached units: an ADU may be created "Using space within an existing dwelling" or by "Building an addition onto an existing dwelling such that the addition is made part of the principal dwelling structure or is attached to the principal dwelling". Basement, attic and garage conversions therefore qualify, and Sec. 27-1577(c)(i) and Sec. 27-1578(h)(vi) let the Code Enforcement Officer waive the surveyor's mortgage loan inspection plan and any of the neighborhood-design standards when the work is entirely internal to an existing structure, which makes an interior unit the cheapest path through the permit. South Portland's ADU definition in Sec. 27-201 reaches further than most: the host may be a single-family detached unit or a primary dwelling unit inside a multi-unit structure of up to three dwelling units, so duplexes and triplexes can add one. If an attached ADU creates a third dwelling unit inside a single building, the city's own application form warns that a sprinkler system may be required.

Sec. 27-1578(b) sets a lot-size split at 10,890 square feet (a quarter acre). Below that, the ADU's modified floor area may not exceed 800 square feet or 90% of the primary dwelling's modified floor area, whichever is smaller. At or above it, the cap is the smallest of 10% of the parcel area, 1,200 square feet, and 90% of the primary dwelling, so a half-acre lot reaches only about 1,089 square feet and it takes roughly 27,600 square feet of land to unlock the full 1,200. On a duplex or triplex the comparison is to the largest legal unit existing when the application is filed. The floor is 190 square feet, matching the statewide minimum in 30-A M.R.S. § 4364-B(6). A separate footprint rule in Sec. 27-1578(c) caps new building coverage created for a ground-floor ADU at 800 square feet for that portion of the building, and detached units are limited to 28 feet in height by Sec. 27-1578(h)(iv). "Modified floor area" is a defined term (Sec. 27-201): gross floor area minus unconditioned space and minus attics and garages whether conditioned or not, so an unheated garage bay under a unit does not count against the cap. That measuring standard is new; before the 2026 amendments these caps were written in terms of "living area".

South Portland repealed its owner-occupancy requirement in 2026. Sec. 27-1578(g) now reads "[RESERVED]"; in the codification dated December 23, 2025 the same subsection read "The owner of the property must occupy either the primary dwelling unit or the ADU", and the city's April 2023 ADU Guidebook still lists that rule. The repeal landed in one of the two amendments credited to Sec. 27-1578 since then, Ord. No. 15-25/26 (1/20/2026) or Ord. No. 25-25/26 (6/2/2026). What survives is an ownership rule, not an occupancy rule: Sec. 27-1578(f) requires the dwellings and the ADU to be held "by a common ownership in part or in whole" and forbids selling the ADU away from the principal dwelling, and that subsection was itself loosened in 2026 from the old "held in the same ownership" language to permit partial or shared ownership. Watch the paperwork rather than the code: the city's current zoning permit form, last revised March 2025, still asks "Will the owner occupy the primary dwelling unit or the ADU?" and restates the repealed sentence beneath it. State law will reach the same result independently, but not yet: 30-A M.R.S. § 4364-B(4)(E), enacted by PL 2025, c. 385, § 17, forbids municipalities from conditioning an ADU on the owner living on the lot, and PL 2025, c. 385, § 23 as amended by PL 2025, c. 733, § 25 applies that mandate to municipalities beginning July 1, 2027.

Sec. 27-1578(i) is a single sentence: "No additional parking spaces shall be required for an ADU." The city's ADU page repeats it in plain language, listing "no requirement for additional parking" among the reasons an ADU is now easier to build. The application form still asks the applicant to show the number and location of parking spaces on the site plan, but that is a disclosure, not a quota. ADUs are also exempt from the electric-vehicle charging infrastructure requirements that otherwise attach to new parking under Sec. 27-1558(4). South Portland is comfortably inside the statewide floor here: 30-A M.R.S. § 4364-B(4)(C), enacted with the original LD 2003 in 2021 and unchanged since, bars any parking requirement for an ADU beyond what the single-family dwelling on the lot already needs, and that provision has bound South Portland since the January 1, 2024 implementation date for municipalities whose ordinances are enacted by the municipal officers.

Sec. 27-1577(a): "Accessory dwelling units (ADUs) require permit approval by the Code Enforcement Officer." ADUs are a P (permitted) use, never an S (special exception), in every district in Chapter 27 that allows dwellings, so there is no discretionary hearing and no Planning Board review in the ordinary case. The city says so itself, advertising "a simplified permitting process (no public hearing in most cases)". The exceptions are narrow: a property already carrying a Planning Board site plan needs a minor amendment under Sec. 27-142, which is handled on the Board's consent calendar, and an ADU bundled with another project that independently needs Board approval may be reviewed with it as one application. The process is a three-stage zoning permit totalling $500: a required 30-minute pre-application meeting, a $200 one-hour application meeting, then a $300 submittal fee, with review running about three to four weeks and a separate building permit taking four to six. Approval lapses if construction has not started within three years (Sec. 27-1578(m)). Note that before Ordinance #4-22/23 in October 2022 an ADU was a Planning Board special exception, which is why an obsolete special exception application form is still findable through the city's site search.

Renting an ADU by the month or year is unrestricted, and the city's guidebook treats long-term rental income as the point of the program. Short-term rental is the hard line, and it is unusually broad. Sec. 27-1577(b) provides that no ADU approved on or after Ordinance #4-22/23 (October 18, 2022) "shall be used, occupied or registered as a short-term rental", and the ban is enforced three ways: the owner must certify understanding of it at application (Sec. 27-1577(c)(vi)), the condition is written into the building permit (Sec. 27-1578(k)), and it is written into the certificate of occupancy, where it "shall run with the property and be binding on and inure to the benefit of the property owner, their heirs, successors and assigns", with the Code Enforcement Officer entitled to inspect and to suspend or revoke the certificate (Sec. 27-1578(l)). Critically, the ban is not limited to the ADU: the owner must certify that neither the single-family dwelling nor the ADU, and on a two- or three-unit property none of the units, may be registered as a short-term rental. Chapter 14, Article XVII carries the same rule from the licensing side: Sec. 14-803(i) bars any "Post-Ordinance #4-22/23 ADU" from being registered as a Hosted Home Stay or Non-Hosted Home Stay, and extends that bar to the other dwelling units on the property. Only grandfathered units escape, and only they may register: Sec. 14-801 defines an "authorized accessory dwelling unit" as one the Planning Board approved under the 2004-to-2022 ADU provisions with a valid certificate of occupancy, or one with a certificate of occupancy issued before February 22, 2004. Short-term rental means stays of less than 30 consecutive days; registration with the City Clerk has been mandatory since January 1, 2019 and must be renewed annually. The ADU also may not be sold away from the principal dwelling (Sec. 27-1578(f)).