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: published 2026 estimates for the Spokane area put a new detached ADU at about 110,000 to 260,000 dollars, an attached ADU or addition at 130,000 to 260,000 dollars, a garage conversion at 50,000 to 120,000 dollars, a basement or interior ADU at 95,000 to 210,000 dollars, and a prefab or modular unit installed at 80,000 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 come from two third-party estimator sites, ADU Zoning and ADU Wizard, both read in full on 2026-08-17 and re-read on 2026-08-24, and neither is a government publication. The fee figures are cited to each jurisdiction's own schedule. The two sources land in nearly the same place on total project cost, which is why totals are shown here, and they differ by roughly a factor of two on cost per square foot, so a per-square-foot rate is not a reliable way to budget an ADU here and none is quoted. Use these to plan, and get an actual price from a builder who walks your lot.
Typical ADU cost in Spokane by type
| ADU type | Published Spokane-area range (2026) | Notes |
|---|---|---|
| New detached ADU, site-built | 110,000 to 260,000 dollars | Most design freedom, usually the highest cost because everything is new. |
| Prefab or modular ADU, installed | 80,000 to 180,000 dollars | Factory-built and set on a prepared site. |
| Attached ADU or addition | 130,000 to 260,000 dollars | Can share structure and utilities with the main home. |
| Garage conversion | 50,000 to 120,000 dollars | Reuses an existing shell, often a lower-cost path. |
| Basement or interior ADU | 95,000 to 210,000 dollars | Lower if plumbing and egress already exist. |
Published Spokane-area ranges for 2026 from ADU Zoning and ADU Wizard, both third-party estimator sites rather than government publications, read in full on 2026-08-24. Actual cost depends on site conditions, finishes, and utility connections, and is never a quote or a guarantee. Confirm with your 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: 3,475 dollars for a 5/8 inch meter (typical for an ADU), 8,688 dollars for 1 inch, 27,803 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 8,362 dollars. Transportation impact fee for an ADU, from the city's 2026 rate schedule effective January 1, 2026 (SMC 17D.075), depends on your impact fee district: 103.58 dollars Downtown, 316.67 dollars Northeast, 379.19 dollars Northwest, 1,283.57 dollars South, 1,847.10 dollars West Plains, 3,266.65 dollars Latah. 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 | Spokane Valley charges transportation impact fees in three subareas adopted by Ordinance 22-005: South Barker Corridor, Mirabeau, and North Pines Road. Neither the city's ADU page nor its impact fee page publishes an ADU rate or an ADU exemption, so ask the Permit Center whether a fee applies to your parcel. Other permit and connection fees follow the city master fee schedule (Resolution 25-013). |
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 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. Published 2026 estimates for the Spokane area put a new detached ADU at 110,000 to 260,000 dollars, an attached ADU or addition at 130,000 to 260,000 dollars, a garage conversion at 50,000 to 120,000 dollars, a basement or interior ADU at 95,000 to 210,000 dollars, and a prefab or modular unit installed at 80,000 to 180,000 dollars. Those come from two third-party estimator sites, not a government publication. 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 8,362 dollars, and the ADU transportation impact fee runs from 103.58 dollars Downtown to 3,266.65 dollars in the Latah District, so your impact fee district matters. This page breaks both down. Spokane Valley charges transportation impact fees in three subareas and publishes no ADU rate or exemption, so ask its Permit Center, 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 2026 impact fee rate schedules, effective January 1, 2026 (static.spokanecity.org) Per-district ADU transportation impact fee rates.
- City of Spokane Valley residential building permits (spokanevalleywa.gov) Spokane Valley master fee schedule (Resolution 25-013).
- City of Spokane Valley transportation impact fees (spokanevalleywa.gov) The three subareas adopted by Ordinance 22-005. No ADU rate or exemption is published.
- ADU Zoning, Spokane builders and costs (aduzoning.org) Third-party estimator, page last verified 2026-07-07.
- ADU Wizard, Spokane ADU costs (aduwizard.com) Third-party estimator, updated for 2026 budgeting.
- 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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