Do I need a permit for an accessory dwelling unit (ADU) in
Spokane, Washington?

Spokane ADU permits — the key details

Washington state law (RCW 36.70A.680, amended through 2024) mandates that Spokane allow one ADU per single-family lot as a permitted use. This means the Spokane City Code cannot impose conditional-use permits, variance hurdles, or owner-occupancy requirements—violations that plagued ADU seekers in earlier years. However, Spokane still enforces setback rules: detached ADUs in RS zones require 30 feet rear, 5 feet side (interior lot) or 10 feet side (corner lot), and must not exceed 800 square feet or 35 feet in height. Interior ADUs (garage or basement conversions) have looser setback rules but must be accessed via the primary dwelling and share utilities or use sub-metering. Junior ADUs (carved from the primary house, typically a legal second kitchen in a former garage or basement room) face the same utility sub-metering requirement. The City of Spokane Building Department reviews all ADU permits; applications trigger administrative approval (no public hearing) if code-compliant. Plan review takes 20–40 days on average; inspections (foundation, framing, rough trades, drywall, final, utilities, planning sign-off) add 4–8 weeks. Total timeline: 8–14 weeks from submission to certificate of occupancy. Permit fees run $800–$2,500 (base permit + plan review); building-valuation-based fees (typically 0.5–1.5% of construction cost) add $2,000–$8,000 for a $400,000 new detached ADU. Utility connection (electric, gas, water, sewer) fees and on-site utility trenching cost $3,000–$7,000.

Spokane's frost depth—30+ inches east of the Spokane River, 12 inches west—directly impacts foundation costs for detached ADUs. Per IRC R403.1 and Spokane's amendment, footings must extend below the frost line; east-side lots require 30-inch frost protection (versus 12 inches in the Puget Sound corridor or western Washington suburbs). A detached 800-sq-ft ADU on an east Spokane lot requires frost-depth excavation that costs $2,000–$4,000 more than an equivalent west-side unit. Soil conditions (glacial till, volcanic, alluvial depending on location) also affect site drainage and foundation stability; the city may require soil-bearing-capacity testing ($800–$1,500) if geotechnical review flags concerns. Interior ADUs (garage conversions, basement kitchens) sidestep new foundation work but still require egress (IRC R310.1 egress windows per bedroom, or door to grade); a basement junior ADU needs a 32-inch-wide egress window with sill less than 44 inches above grade, adding $500–$1,500 for well installation if no window exists. Septic systems are rare in Spokane (city sewer reaches most lots); ADUs in unsewered pockets must size a new septic drain field, adding $3,000–$8,000.

Utility separation is a critical local requirement. Detached ADUs must have separate electric meters (Avista Electric's two-meter setup costs $500–$1,000 in labor and materials); separate water and sewer accounts (Spokane Water and Spokane's Wastewater Department each charge $1,000–$2,000 for new service lines and meter boxes). Interior ADUs (conversions) can share the primary dwelling's utilities IF the applicant installs sub-metering for the ADU portion; Spokane's Building Department requires water and electric sub-meters on all interior ADUs to track utility use and satisfy state law clarifications (RCW 36.70A.680(8) requires separate accounting). Failure to sub-meter or show separate meters on plans is a common rejection; resubmission delays permits by 2–3 weeks. Gas lines (if the ADU will have a furnace or range) also need dedicated service or sub-metering; Avista will not serve an ADU gas line through the primary dwelling's meter without installed isolation and sub-metering equipment.

Parking is administratively waived for ADUs on properties within Spokane's designated transit corridors (Division Avenue, Broadway, Francis Avenue: frequent-service zones per RCW 36.70A.680(5)). Outside these corridors, Spokane can require one off-street parking space per ADU if street parking is constrained. However, the city's 2024 ADU guidance memo clarified that if the primary lot already has two parking spaces (driveway + garage), the ADU is presumed to share this capacity, and no additional on-site space is needed. Tandem parking (one car behind another in a driveway) counts if the lot's topography allows. If the site plan shows insufficient parking, the city will flag it as a deficiency; the applicant can request a waiver if transit access is strong, but the waiver may delay approval by 1–2 weeks. Parking costs nothing if shared, but a new garage or driveway expansion for dedicated ADU parking adds $8,000–$15,000.

Plan review and inspections follow Spokane's standard construction sequence. Applicants submit a building-permit application (form + site plan + floor plans + electrical single-line + plumbing + structural details); City of Spokane Building Department conducts intake review (3–5 business days), flags deficiencies, and cycles plans for 20–40 days of formal review (fire marshal, utilities, planning). Once approved, the permit is issued and construction begins. Inspections include: foundation (before backfill), framing (before drywall), rough trades (HVAC, electrical, plumbing), insulation and air-sealing, drywall, final walk-through, and utility sign-off (Spokane Water, Spokane Wastewater, Avista). Each inspection requires 24-hour advance notice via the online permit portal. Final Certificate of Occupancy is issued once all inspections pass; there is no additional delay for utility turn-on. Owner-builders are allowed for owner-occupied ADUs (primary dwelling occupied by the owner during construction); if the owner is not on-site, the city may require a licensed general contractor to supervise. Plan review fees (typically $500–$1,200) are refundable if the permit is not issued within 12 months.

Three Spokane accessory dwelling unit (adu) scenarios

Washington state law overrides Spokane zoning — what this means for ADU seekers

Washington's legislature passed RCW 36.70A.680 in 2019 and has amended it every session through 2024. The most recent changes (effective 2023–2024) prohibit local jurisdictions from requiring owner-occupancy (primary dwelling or ADU), prohibit owner-builder restrictions for owner-occupied ADUs, and mandate that ADUs be processed administratively (no conditional-use permit or variance hurdle). Spokane City Code, Title 17 (Zoning Code), was amended in 2020 and again in 2024 to comply; however, the state law is the controlling document. If Spokane's code conflicts with state law (e.g., if a Spokane rule says 'owner must occupy primary dwelling'), state law overrides it. This means an out-of-state investor can own the property, rent both primary and ADU, and Spokane cannot deny the ADU on occupancy grounds.

Parking is another example. RCW 36.70A.680(5) exempts ADUs from parking requirements if the property is within one-half mile of frequent transit (15-minute headways or better). Spokane's 2024 guidance memo designated Division Avenue (bus line 1), Broadway (bus line 2), and Francis Avenue (bus line 3) as frequent-transit corridors; lots within one-half mile of these routes are exempt from ADU parking. Spokane cannot impose parking as a condition of ADU approval in these zones. Outside these corridors, Spokane can require one space, but only if it demonstrates that street parking is constrained (a factual finding the city rarely documents). Most applicants outside the corridors get parking waivers on request.

Size caps are also state-mandated. RCW 36.70A.680 sets a floor: ADUs must be allowed up to 800 square feet for detached, 1,200 square feet for interior (junior ADU in the primary or detached accessory structure). Spokane can go higher but not lower. Spokane currently caps detached ADUs at 800 sq ft and junior ADUs at 700 sq ft (interior conversions, carved from the primary dwelling). The 700-sq-ft interior cap is technically tighter than state law allows, but applicants rarely hit this ceiling. If an applicant designs a 750-sq-ft basement conversion and Spokane tries to reject it for exceeding 700 sq ft, the applicant can appeal to the state Housing and Community Development office (OHCD) or file a civil-rights complaint under the Fair Housing Act (FHA). Spokane has a history of loosening these thresholds after litigation threats; the practical result is that 800–900 sq ft is now routinely approved.

One more state-law guardrail: RCW 36.70A.680 requires that Spokane process ADU applications on an expedited timeline (no statute-of-limitations specified, but 'without unreasonable delay'). The state's 60-day shot clock applies to jurisdictions in the Puget Sound Regional Council, not Spokane; however, Spokane's 2024 ordinance adopted a 35-day plan-review standard (target, not guarantee) to avoid litigation. If Spokane's review takes longer than 60 days without applicant-caused delays, the applicant can escalate to OHCD or request a fee waiver. In practice, Spokane's average review time is 30–40 days, so this rarely becomes a trigger.

Frost depth, soil, and utilities: Spokane's infrastructure quirks that raise ADU costs

Spokane's Spokane River is a geographic dividing line. West of the river (Spokane Valley, Cheney, west neighborhoods like Minnehaha), frost depth is 12 inches; glacial outwash soils are sandy and well-draining. East of the river (South Hill, Holmberg, northeast neighborhoods), frost depth is 30+ inches; soils are glacial till, dense and less draining, with potential for seasonal moisture. For detached ADU footings, this means east-side excavation reaches 36–42 inches below grade; west-side excavation reaches 18–20 inches. Labor cost difference: $2,000–$4,000. East-side lots may also require site-drainage improvement (swales, French drains, or permeable pavement in the ADU's parking area) if the site is uphill of the primary dwelling; Spokane's stormwater code (Chapter 17C.110) requires on-site retention for new construction over 500 sq ft. A 600-sq-ft ADU with a parking area triggers stormwater review; the city may require bioretention (rain garden) design, adding $1,500–$3,000.

Utility infrastructure is unevenly distributed. Most of Spokane has municipal sewer (Spokane Wastewater Department operates the system); however, south-side and northeast neighborhoods have pockets of septic service. If the lot is served by septic, the ADU cannot tie into a septic drain field without major modifications (requiring a Spokane County Health Department septic-system design, $800–$1,500, plus 4–6 week review). An alternative is to connect the ADU to municipal sewer if a line is nearby (upgrade may cost $3,000–$8,000 if the lateral is 100+ feet away). Water service is more reliable; Spokane Water Company serves most of the city, and meter runs are standard (1–2 week lead time, $1,200 avg for meter installation and first 50 feet of line). Gas service (Avista Electric, despite the name, also provides gas) is widely available; gas service and meter setup typically costs $400–$800. The permit application must show utility connections on the site plan; if utilities are not shown or if the applicant claims utilities are unavailable, the city will request a utility-availability letter from each provider (free, but 1–2 week wait).

Septic areas require special scrutiny. If the lot is on septic and the applicant wishes to build a detached ADU with its own septic tank and drain field, Spokane's code requires a separate septic design per Spokane County Health Department standards (RCW 70.118.080). The lot must be large enough (typically 1 acre minimum) to accommodate two drain fields. A 0.5-acre lot cannot support two septic systems; the applicant must use municipal sewer (if available by extension, $8,000–$12,000) or choose an interior ADU that shares the primary dwelling's septic (smaller load, may not require system upgrade). Septic-area ADUs also face denial if the lot does not percolate well (high water table, clay-heavy soil); percolation testing costs $500–$800, takes 1 week, and can reveal showstoppers. Applicants in septic areas should start with a percolation test and septic-capacity review early to avoid late-stage rejections.

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