Renology Cost Guide · Seattle, Washington
ADU (Accessory Dwelling Unit) Cost in Seattle: 2026 Guide
How much a ADU build costs in Seattle, Washington in 2026: typical range $180,000-$320,000, what drives the price, permit basics, resale ROI, and when you need a licensed pro.
$180,000-$320,000
Typical 2026 project range
Seattle
Washington
May 2026
Last reviewed
Quick answer
A ADU build in Seattle, Washington typically costs $180,000-$320,000 in 2026. An accessory dwelling unit is a self-contained second home on a residential lot, either detached, attached, or a garage conversion. Cost is driven by whether utilities already reach the site and by local size and setback rules.
A ADU build in Seattle, Washington typically costs $180,000-$320,000 in 2026, depending on scope and finish level. This guide breaks down what drives that price, the permit basics for Seattle, what the project recoups at resale, and when you need a licensed contractor.
ADU (Accessory Dwelling Unit) cost in Seattle (2026)
These are planning benchmarks for Seattle, Washington, reconstructed from Renology’s published cost data for the Washington market. Treat them as a starting point, then confirm against local bids.
| Scope | Typical range in Seattle |
|---|---|
| Budget / minor | Around $180,000 and up |
| Mid-range | $180,000-$320,000 |
| High-end / upscale | Up to $320,000 and beyond for premium scope |
Planning benchmarks, not quotes. Source: Renology Cost Index 2026.
What drives ADU build cost in Seattle
- Type: garage conversion (cheapest), attached addition, or detached new build (most expensive)
- How far power, water, and sewer have to run to reach the unit
- Foundation and site work, which climbs on sloped or tight lots
- Kitchen and bath fixtures, since an ADU is a full dwelling with its own utilities
- Local size caps, parking rules, and impact fees, which vary by city even within the same state
Permits for a ADU build in Seattle: Seattle Department of Construction and Inspections (SDCI)
In Seattle, ADU build permits are issued by the Seattle Department of Construction and Inspections (SDCI). Seattle SDCI issues permits through the Seattle Services Portal. Many smaller remodels qualify as a Subject-to-Field-Inspection (STFI) permit, which needs no pre-submittal appointment, while larger additions and alterations follow the standard Addition or Alteration permit path. You create a portal account to apply and to schedule inspections.
Typical timeline: Seattle does not publish one fixed approval time for all remodels. Subject-to-Field-Inspection permits for qualifying small projects are the faster path because plan details are verified in the field, while full addition or alteration permits take longer. Confirm your project type with SDCI.
Local note: Seattle offers a Subject-to-Field-Inspection track for qualifying small remodels, where the work is checked by an inspector in the field instead of through full up-front plan review, which is a distinct SDCI process not offered the same way in the California cities on this list.
An ADU always requires a building permit and a plan review, and usually separate electrical, plumbing, and mechanical permits. California has loosened statewide ADU rules to speed approvals, but each city still sets size, setback, and parking specifics. Start with the local planning department early, because ADU approvals depend heavily on lot-specific zoning.
Resale ROI: what a ADU build recoups
Cost vs. Value added accessory dwelling units to its 2026 report, but reported that ADU returns vary widely by market. California’s loosened ADU rules can add significant rentable value, so resale recoup is one of several reasons homeowners build.
When you need a licensed pro
An ADU is a full dwelling and should always be built by a licensed general contractor with a design professional on plans. This is not a homeowner-managed project: it involves foundation, utilities, and code-compliant egress that all get inspected.
Questions to ask a Seattle contractor
- Does the quote include running utilities to the unit, or only the structure?
- Have you built ADUs under this city’s current rules, and what size can this lot support?
- What is the realistic permit timeline here, and is design and plan review included?
Renology planning tools
Run your own Seattle numbers
Compare this project against every other renovation type and see what comes back at resale.
Seattle Permit Watch
See what Seattle homeowners are actually building
Our Permit Watch tracks residential remodel permits filed with the City of Seattle each month, by neighborhood, from public SDCI data. It is a live read on demand, cost, and the projects going up near you.
Go deeper
For before/after projects, timelines, and local trends, read the full ADU (Accessory Dwelling Unit) guide for Seattle.
Frequently asked questions
How much does a ADU build cost in Seattle in 2026?
What drives the cost of a ADU build the most?
Do I need a permit for a ADU build in Seattle?
Does a ADU build add resale value?
Do I need a licensed contractor for a ADU build?
Renology ROI Calculator
What this project recoups at resale in Seattle
Adjust the scope to see how much of the cost comes back at resale, using local cost ranges and cost-recouped rates cited from Renology’s 2026 Renovation ROI Calculator.
Standard remodel with mid-grade materials
ADU / Garage in Seattle, midrange scope
2026 cost range
$180,000 to $320,000
Resale value is an estimate based on national Cost vs. Value recoup data and local cost ranges; actual return varies by home, market, and timing. Planning benchmarks, not bids.