Auburn, WA Accessory Structures: ADU Rules (2026)

010(A) makes accessory dwelling units permitted outright, not conditional or discretionary, in every residential zone that allows single-unit detached or middle housing, whether built with new construction or added to an existing home. Subsection (B) allows two ADUs per residential unit on individually owned lots that meet the zone's minimum lot size, and subsection (C) lets those two units be attached, detached, or one of each. Size limits in subsection (D) run from a 300-square-foot floor to a 1,000-square-foot ceiling, with no more than two bedrooms per unit, and garage space does not count toward that square footage.

Exterior appearance rules require that alterations not change the building's look from a single-unit detached dwelling, exterior stairs must sit in a rear or side setback at least 10 feet from an adjoining lot line, and a garage converted to living space must get a replacement door matching the house's exterior. Parking is waived entirely for an ADU within a half-mile walk of a major transit stop; beyond that distance, one on-site parking space per ADU is required. An ADU can be sold as a separate condominium unit or through unit lot subdivision, but only with a separate water and sewer connection, and every ADU must tie into municipal sewer or an approved septic system.

Before final inspection, the owner must record a statement with the county putting future owners on notice that occupying or renting the ADU requires ongoing compliance with the Auburn Zoning Code; removing an ADU requires city permits and inspections before that recorded notice can be released.