ADU rules in Auburn

Detached backyard ADUs are allowed outright. ACC 18.32.010(A) permits ADUs in all residential zones that allow single-unit detached or middle housing, and the use table at ACC 18.07.020 marks accessory dwelling units 'P' (permitted) in every residential zone: RC, R-1, R-2, R-3, R-4, R-NM and R-F. Subsection (C) lets both allowed ADUs be detached, either as one or two structures, and subsection (I) lets a detached ADU sit on the rear lot line where that line abuts a public alley, unless the city routinely plows snow in the alley. Converting a legally established structure that does not meet current zoning, a detached garage for example, does not trigger a requirement to bring it into conformance (subsection (J)).

Attached and internal ADUs (basement units, additions, garage conversions) are allowed on the same outright basis as detached ones. The definition at ACC 18.04.018 covers a self-contained unit with its own bathroom, kitchen, living and sleeping areas that 'may be attached to or detached from' the single-unit detached or middle housing unit, and ACC 18.32.010(C) says the two permitted ADUs 'may be attached, detached, or any combination thereof.' What bites here is design, not use: alterations must not change the appearance from that of a single-unit detached dwelling as determined by the planning director, exterior stairs must be in a rear or side setback and no closer than 10 feet to an adjoining property line, and a converted garage door must be replaced with materials matching the house exterior (ACC 18.32.010(E)).

ACC 18.32.010(D) sets a flat cap: an ADU may not exceed 1,000 square feet, may not be smaller than 300 square feet, and may not have more than two bedrooms, with garage space excluded from the calculation. The 1,000 square foot ceiling matches exactly the floor imposed by RCW 36.70A.681(1)(f), which bars a maximum gross floor area below 1,000 square feet. Chapter 18.32 sets no ADU-specific height limit and instead applies the underlying zone's development standards; ACC 18.07.030 caps accessory buildings and structures at 24 feet in R-2, R-3, R-4, R-NM and R-F and at 35 feet in RC and R-1, while principal buildings run from 35 to 75 feet by zone. Separately, ADUs count against the lot's unit total, which is four base units per lot in every residential zone (six with the transit or affordability bonus).

Nothing in Chapter 18.32 ACC requires the owner to live in the ADU or in the main house. Auburn's old owner-occupancy rule lived in ACC 18.31.120, which Ordinance 6959 repealed in 2024 when it created Chapter 18.32; a full-text search of the current code turns up no owner-occupancy condition attached to ADUs. RCW 36.70A.681(1)(b) independently bars Auburn from requiring the owner of a lot with an ADU to occupy the ADU or any other unit on the lot. The city does still require a disclosure statement recorded with the county before an ADU is finaled (ACC 18.32.010(K)(2)), but the current form (City of Auburn ADU Disclosure Statement, CD-Planning revision 3/13/26) is a notice to future owners only; unlike the pre-2024 version it contains no affidavit of owner occupancy.

ACC 18.32.010(F) is a two-line rule: no off-street parking is required for an ADU within one-half mile walking distance of a major transit stop, and one on-site space per ADU is required beyond that distance. 'Major transit stop' is defined at ACC 18.04.597 as a stop on a high capacity transportation system funded or expanded under Chapter 81.104 RCW, a commuter rail stop, a stop on a rail or fixed guideway system, or a stop on a bus rapid transit route. Middle housing units are governed by the identical standard in ACC 18.25.050, and on-site spaces may be garage or driveway spaces. The one-space figure sits within the ceilings in RCW 36.70A.681(2)(a). Note that the city's ADU Information Sheet still describes the transit exemption as 'subject to the discretion of the planning director or designee' and cites the superseded RCW 36.70A.698; the adopted code grants no such discretion.

An ADU is a permitted (P) use in all seven residential zones under ACC 18.07.020, with no conditional use permit, administrative use permit or public hearing. ACC 18.32.010(K)(1) sets the process: the owner applies for an ADU permit with a completed application form, floor and structural plans, and fees. That runs as a building permit, which ACC 14.03.010(A) classifies as a Type I administrative decision not subject to SEPA review. The city's ADU Information Sheet routes applicants to MyBuildingPermit.com and recommends, but does not require, a pre-application meeting. Before the ADU is finaled the owner must record the disclosure statement in ACC 18.32.010(K)(2), and later removal of an ADU also requires permits, inspections and a recorded release (subsection (L)).

Chapter 18.32 ACC places no limit on renting an ADU: there is no owner-occupancy test, no minimum lease term and no ADU-specific occupancy cap, and the recorded disclosure statement in ACC 18.32.010(K)(2) expressly contemplates the unit being 'occupied or rented.' Any rental unit in Auburn needs a rental housing business license under Chapter 5.22 ACC, which by ACC 5.22.010 'applies to all rental units in the city of Auburn.' Short-term rental is treated as a separate land use rather than an ADU rule: ACC 18.07.020 lists 'Bed and breakfast or short-term rentals' as permitted (P) in RC, R-1, R-2, R-3, R-4 and R-F and not permitted (X) in R-NM. Auburn has adopted no ADU-specific short-term-rental ban, although RCW 36.70A.680(5)(a) would allow one. An ADU may also be conveyed away from the main house: ACC 18.32.010(G) allows sale as a condominium unit or as a separate parcel through the unit lot subdivision process in Chapter 17.27 ACC, in which case separate water and sewer connections are required.