Portland, ME Accessory Structures: ADU Rules (2026)

2(A) of the Land Use Code, Article 6 (Accessory Uses), governs ADUs citywide. ADUs may be added as new detached structures, as additions to the principal building, or by converting space within an existing lawfully conforming or nonconforming structure, but converting a nonconforming structure can't increase its degree of nonconformity, and the design must consider the privacy of adjacent properties where practicable. A property can host up to two ADUs. Size is capped relative to the principal dwelling: a single ADU's gross floor area is limited to two-thirds of the largest principal unit's gross floor area, and where a lot has more than one detached ADU, their combined square footage cannot exceed the gross floor area of the principal structure itself.

ADUs are exempt from the Land Use Code's maximum lot coverage and minimum lot area per dwelling unit standards, removing two common obstacles to adding a unit on a tight urban lot. Height for a detached ADU is capped at 18 feet, rising to 25 feet if the unit sits above a garage. In residential zones, detached ADUs of any size must meet the Article 7 side and rear setbacks that apply to detached accessory structures under 250 square feet, and a detached ADU cannot be sited between the principal structure and the front lot line. These standards took effect with the Land Use Code's July 1, 2026 effective date, codified as Chapter 14 of the Portland Code of Ordinances.