Guide
How much does an ADU cost in Spokane? (2026 guide)
ADU cost is a range, not a single price. It depends on the ADU type, size, your site, and the finishes you choose. This guide breaks the cost into the pieces that actually move the number, with dated Spokane-specific ranges and fee figures cited to their sources.
Short answer: as of 2026, a new detached ADU in the Spokane area typically runs about 150,000 to 400,000 dollars, an attached ADU or addition 130,000 to 300,000 dollars, a garage conversion 100,000 to 200,000 dollars, and a basement or junior ADU 60,000 to 180,000 dollars. Cost is always a range that depends on your site, size, and finishes, never a quote. The rest of this guide breaks down what moves the number, including the Spokane permit fees and system development charges most estimates leave out.
How to read these numbers
Every dollar figure here is a dated range with a stated basis, not a quote or a guarantee. The construction ranges below are typical Spokane-area figures as of 2026, and the fee figures are cited to each jurisdiction's schedule. Use these to plan, and get an actual price from the licensed local builder you are matched with.
Typical ADU cost in Spokane by type
| ADU type | Typical Spokane-area range (2026) | Notes |
|---|---|---|
| New detached ADU, site-built | 150,000 to 400,000 dollars (about 250 to 400 dollars per sq ft) | Most design freedom, usually the highest cost because everything is new. |
| New ADU above a garage | 200,000 to 300,000 dollars or more | A unit built over a new or existing garage. |
| Attached ADU or addition | 130,000 to 300,000 dollars (about 200 to 350 dollars per sq ft) | Can share structure and utilities with the main home. |
| Garage conversion | 100,000 to 200,000 dollars | Reuses an existing shell, often a lower-cost path. |
| Basement ADU or JADU | 60,000 to 180,000 dollars | Lower if plumbing and egress already exist. |
Typical Spokane-area ranges as of 2026. Actual cost depends on site conditions, finishes, and utility connections, and is never a quote or a guarantee. Confirm with your matched builder.
What drives ADU cost
Two ADUs of the same size can differ widely in price. These are the factors that move the number the most:
- New construction versus conversion. Building new costs more than adapting existing space.
- Site work. Slope, soil, trees, demolition, and access affect excavation and foundation cost.
- Utilities. The distance and difficulty of running water, sewer or septic, and power to the unit is a major variable. Septic capacity can be decisive on county lots.
- Foundation and structure. A new slab or crawlspace, and any structural upgrades for a conversion.
- Size and layout. More square footage, and more plumbing locations, raise cost.
- Finish level. Flooring, cabinets, counters, and fixtures span a wide range.
- Design and engineering. Plans, structural engineering, and energy calculations.
- Permit fees and system development charges. These vary by jurisdiction.
Permit fees and system development charges by jurisdiction
In Washington, the charge people often call a system development charge is the general facilities charge (GFC) for water and sewer, plus the transportation impact fee. These depend on the jurisdiction and the utilities that connect. HB 1337 caps ADU impact fees at 50 percent of what the principal unit would pay (RCW 36.70A.681).
| Jurisdiction | Permit fees and charges (2026) |
|---|---|
| City of Spokane | Water GFC: 3,366 dollars for a 5/8 inch meter (typical for an ADU), 8,415 dollars for 1 inch, 26,930 dollars for 2 inch. Sewer GFC (2026): 4,887 dollars for a 5/8 inch meter (SMC 13.03.0734), so a typical ADU's water and sewer general facilities charges total about 8,253 dollars. Transportation impact fee for an ADU: about 106 dollars in the high-density transit core, up to roughly 1,000 dollars farther out (SMC 17D.075). Building permit fee is valuation-based, so use the city estimator. |
| Unincorporated Spokane County | Confirm current ADU permit fees with Spokane County Building and Planning. |
| Spokane Valley | Exempts ADUs from impact fees. Other permit and connection fees follow the city master fee schedule; confirm with the city. |
Soft costs people forget
- Design and architectural plans.
- Structural and civil engineering.
- Surveys and site plans.
- Utility connection or capacity fees.
- Financing costs, if you borrow. See our Washington ADU financing guide.
Finish-level detail
Interior finishes span a wide range. Budget-grade flooring, cabinetry, and fixtures keep a project at the low end of these ranges, while premium finishes push toward the high end. Get a fixed quote from your matched builder for your finish choices.
ADU cost questions
- How much does an ADU cost in Spokane, WA?
- The cost depends on the ADU type, size, site conditions, utilities, and finishes, so it is a range rather than a single number. As of 2026, a new detached ADU in the Spokane area typically runs 150,000 to 400,000 dollars, an attached ADU or addition 130,000 to 300,000 dollars, a garage conversion 100,000 to 200,000 dollars, and a basement ADU or JADU 60,000 to 180,000 dollars. Cost is always a range, never a guaranteed quote.
- What drives the cost of an ADU the most?
- The biggest drivers are ADU type (new construction versus converting existing space), size, site work and utility connections, foundation and structural needs, and finish level. Sloped lots, long utility runs, and septic capacity can add significant cost on some properties.
- Is a garage conversion cheaper than a new detached ADU?
- Often yes, because a conversion reuses an existing foundation, walls, and roof. The savings vary with the condition of the structure and how much new plumbing, electrical, and insulation the unit needs. See our garage conversion cost guide for detail.
- Do I have to pay system development charges for an ADU?
- System development charges and permit fees depend on your jurisdiction and the utilities involved. In the City of Spokane, the 2026 water and sewer general facilities charges for a typical ADU meter total about 8,253 dollars, and this page breaks them down. Spokane Valley exempts ADUs from impact fees, and unincorporated Spokane County fees are confirmed with Building and Planning, because these charges change and differ across the three jurisdictions.
References and primary sources
Regulatory and fee specifics on this page are cited to the current primary sources below. Confirm any figure against its source for your specific parcel before you rely on it.
- City of Spokane residential fee estimator (my.spokanecity.org) Valuation-based building permit fee for an ADU.
- City of Spokane general facilities charges (my.spokanecity.org) Water and sewer GFC schedule and zone map.
- Spokane County Building and Planning (spokanecounty.gov) County permit fees.
- City of Spokane Valley residential building permits (spokanevalleywa.gov) Spokane Valley master fee schedule; ADUs exempt from impact fees.
- RCW 36.70A.681 (app.leg.wa.gov) HB 1337 caps ADU impact fees at 50 percent of the principal unit.
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