Everything homeowners ask about building an ADU in Spokane — answered with 2026 data.
Yes, ADUs are allowed in All residential zones, RA, R1, R2, RMF and RHD, including planned unit developments; attached or detached (SMC 17C.300.100(A)). In R1, R2, RMF and RHD an ADU may also be added to a lot whose principal structure is non-residential (SMC 17C.300.130(A)(1)(f)). in Spokane, WA. The maximum size for a detached ADU is 1,000 sq ft of floor area (SMC 17C.300.110(A)(2)). Building footprint up to 1,200 sq ft in every residential zone (Table 17C.111.205-2), counted within the lot's total building coverage cap of 50% in RA, 65% in R1, 80% in R2 and 100% in RMF/RHD (Table 17C.111.205-1, note 5). Attached ADUs can be up to 1,000 sq ft, excluding any related garage area. Spokane has an ADU-friendliness score of 8/10. Check our full Spokane ADU rules page for complete zoning details.
ADU construction in Spokane typically costs $130,000-$260,000. The cost per square foot averages $200-$400. Garage conversions are more affordable at $50,000-$120,000, while prefab ADUs cost $135,000-$280,000. Permit fees add About $2,000-$3,200 in city fees for a $150,000-$300,000 ADU (2025 New SFR/ADU fee table: $65 processing, plan review at 50% of the permit fee, $100 stormwater review, 2.5% technology fee, $6.50 state fee). A CPI-adjusted 2026 schedule now applies; confirm with the DSC. to your budget. See our detailed Spokane ADU cost guide.
Detached ADUs in Spokane can be up to 1,000 sq ft of floor area. Attached ADUs are limited to 1,000 sq ft, excluding any related garage area. Junior ADUs (JADUs) have a cap of Not a category in Spokane or Washington law. A basement or attic converted inside the house is simply an attached ADU, and that conversion may exceed the 1,000 sq ft cap (SMC 17C.300.110(A)(1)(c)). Maximum height is 25 ft for a detached ADU in every residential zone (Table 17C.111.205-2, accessory dwelling units rows); the state floor is 24 ft (RCW 36.70A.681(1)(g)). Primary buildings may reach 35 ft in RA and 40 ft in R1 through RHD. You are allowed up to Two ADUs per lot in the RA, R1, R2, RMF and RHD zones, in any mix of attached and detached on a qualifying lot.
Rear setback: 0 ft where the rear lot line abuts an alley; 5 ft where there is no alley (Table 17C.111.205-2). An existing garage or shed already inside a rear or side setback may be converted in place (SMC 17C.300.130(B)(4), 17C.111.240), but any added floor area puts the whole structure under current setbacks. Side setback: Same as the primary structure: 3 ft from an interior side lot line on lots 40 ft wide or less, 5 ft on wider lots, and 5 ft from a street side lot line (Table 17C.111.205-2). An additional angled setback applies near the interior side line (SMC 17C.111.230(C), 17C.111.235(E)); a detached structure's setback may drop to 0 ft with a signed neighbor waiver (SMC 17C.111.240(C)). Front setback: Not listed separately for ADUs in Table 17C.111.205-2. The primary-building front setback is 10 ft in R1, R2, RMF and RHD and 15 ft in RA, and state law bars an ADU setback stricter than the principal unit's (RCW 36.70A.681(1)(h)). SMC 17C.300.130(B)(4)(a) addresses converting an existing detached structure that sits in the front setback; confirm placement with the Planning Services Department. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None. Spokane removed all minimum off-street parking requirements citywide (SMC 17C.230.110, Ordinance C36629, effective March 6, 2025), and SMC 17C.300.130(A)(3) simply refers ADUs to Chapter 17C.230. State law would in any case bar parking mandates within 1/2 mile of a major transit stop (RCW 36.70A.681(2)). Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Not required. The only exception: if the lot also has a short-term rental under Chapter 17C.316 SMC, one unit must be the owner's principal residence for more than six months a year and cannot be rented (SMC 17C.300.110(B)); the ADU approval is then transferable only to a buyer who will owner-occupy (SMC 17C.300.140). Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Spokane is typically The city publishes no ADU-specific review time. State law (RCW 36.70B.080(1)(d)) makes 65 days from a complete application the default for permits that need no public notice, unless Spokane has adopted its own periods; ask the Development Services Center (509-625-6300). Plan check: Plan review is 50% of the building permit fee for new single-family, ADU and duplex permits and is paid at submittal before the plans are routed (2025 Development Services Fee Schedule); for a $200,000 ADU that is $736 of a $2,416 total. Pre-approved plans availability: None found. The city's residential permit pages (August 2026) list an ADU building-permit application but no city pre-approved ADU plan set; the fee schedule offers a stock plan review fee (25% of the job-value fee) for repeat plans, so ask whether a builder's repeat design qualifies.
Average ADU rental income in Spokane: 1-bedroom units rent for $1,050-$1,250/month, 2-bedroom units for $1,300-$1,500/month. Short-term rentals: Allowed with a short-term rental permit under Chapter 17C.316 SMC, but once a lot with an ADU has a short-term rental, one unit must be owner-occupied for more than six months a year and cannot be rented (SMC 17C.300.110. Expected ROI: 5-11% gross rent yield: HUD FY2027 fair market rents of $1,173 (1BR) to $1,494 (2BR) a month on a $130,000-$260,000 build, before vacancy, taxes and upkeep.
The minimum lot size for an ADU in Spokane is None for an ADU: Chapter 17C.300 sets no minimum lot area, so any existing residential lot qualifies (state floor: RCW 36.70A.681(1)(e)). For new subdivisions the minimum lot is 1,200 sq ft in R1 through RHD and 7,200 sq ft in RA (Table 17C.111.205-1). The only size test is for attached ADUs: the house must be at least 1,000 sq ft. This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Spokane, a garage conversion typically costs $50,000-$120,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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