ADU rules in Spokane

Detached backyard ADUs are permitted outright in every Spokane residential zone (RA, R1, R2, RMF, RHD), including inside planned unit developments, and Table 17C.111.115-1 lists Accessory Dwelling Unit as 'P' (permitted) in all five. Two ADUs are allowed per lot. A detached ADU may be built on a site with an existing residential use, built together with a new home, or converted from an existing accessory structure such as a garage; in R1, R2, RMF and RHD it may even go on a site whose principal structure is non-residential. Detached units get their own relaxed siting standards: 25 ft height, 5 ft rear setback or 0 ft where the lot abuts an alley, and side setbacks matching the primary structure.

Attached and internal ADUs are permitted in the same five residential zones with no hearing and no discretionary review, but Spokane gates them on the size of the existing house: the principal structure must have at least 1,000 sq ft of floor area, excluding an attached garage, before an attached ADU can be established. That threshold bites in Spokane's older housing stock because SMC 17A.020.060 defines floor area as the above-ground portion of a building and excludes any floor sitting four feet or more below the adjacent right-of-way, so a full basement generally does not count toward the 1,000 sq ft. Owners of small bungalows can therefore be shut out of a basement or attic conversion and pushed toward a detached unit instead. No such minimum applies to detached ADUs.

The 1,000 sq ft figures came in with ORD C36696 (effective July 11, 2025), replacing the old 800 sq ft attached and 975 sq ft detached caps, and they match the floor RCW 36.70A.681(1)(f) sets for cities once the state ADU mandate binds them. The limits stack, and on a tight lot the practical ceiling is often site coverage rather than the 1,000 sq ft number: building coverage counts the house and every accessory structure together, and R1 and R2 lots are separately held to 60 percent impervious coverage (40 percent inside an Area of Drainage Concern) unless an engineer's stormwater drainage plan is submitted. An attached ADU also counts inside the maximum building footprint per primary building, which is 2,450 sq ft in R1 and R2 on lots of 7,000 sq ft or less and 35 percent on larger ones. Basement and attic conversions are the one route past the 1,000 sq ft ceiling.

No owner occupancy is required for an ordinary ADU, whether the owner lives on site or not, and no deed restriction is recorded any more. The requirement returns only if the lot also holds a short-term rental permit under Chapter 17C.316: then one of the dwelling units on the lot must be occupied by an owner as their permanent and principal residence for more than six months of each calendar year, and that owner may not collect rent on it. On such a lot the ADU approval is not transferable except to a new owner who will occupy it, and the approval expires if the titleholder stops owning and residing in either unit. Owners can ask the director to waive the requirement for up to a one-year absence for good cause such as a job dislocation, sabbatical, education or illness.

SMC 17C.300.130(A)(3) sends ADU parking to Chapter 17C.230, and SMC 17C.230.110(A) states flatly that except as provided in that chapter there is no required minimum number of off-street parking spaces. Spokane deleted its minimums citywide with ORD C36629, effective March 6, 2025, which also rewrote the summary table so the 'minimum required' column reads None for residential household living. The only route back to a minimum is SMC 17C.230.110(B), which lets a required number be attached as a condition of a Conditional Use permit, and ADUs are not a conditional use. Chapter 17C.230 still sets parking maximums, and SMC 17C.300.140(B)(2) keeps a vestigial expiration clause about ceasing to maintain 'the required' spaces, which has no work to do while no spaces are required.

Purely administrative. SMC 17C.300.120(A) says an owner seeking to establish an ADU must obtain a building permit and a certificate of occupancy from the building services department, and that is the whole procedure: no land use application, no hearing, no notice to neighbors, and no recorded deed restriction. Table 17C.111.115-1 lists ADUs as 'P', permitted, in all five residential zones, in contrast to mobile home parks (CU) and short-term rentals (P/CU). The unit must still meet building, electrical, fire and plumbing code and use the principal dwelling's municipal utilities and address.

Chapter 17C.300 imposes no minimum lease term, no cap on rent, and no restriction on renting the ADU or the main house to anyone. Renting for 30 days or more triggers SMC 10.57.020, the residential rental registry adopted in 2023, which requires an annual business license and registration of each unit at $15 per unit per year. For stays under 30 nights, SMC 17C.316.040(B) allows one short-term rental in an accessory dwelling unit, and allows one in the house and one in the ADU at the same time, in both cases subject to the owner occupancy requirement of SMC 17C.300.110(B); the permit requires a city business license, a notarized life safety form, liability insurance under Chapter 64.37 RCW, a neighbor notification letter, and $200 to apply plus $100 a year to renew in residential zones. Spokane's code is silent on selling an ADU separately, and nothing in Chapter 17C.300 prohibits condominiumizing one.