How Much Does an ADU Cost in Washington State in 2026?
Most ADUs in Washington State cost between $100,000 and $280,000. Where you land in that range depends on where you’re building, what type of ADU, and what the site looks like.
Here’s a straight breakdown for Clark County homeowners.
ADU Cost by Type in Washington State (2026)
Garage Conversion ADU: $60,000–$120,000
The lowest-cost path to an ADU, assuming you’ve got a garage worth converting. You’re adding insulation, HVAC, electrical upgrades, plumbing, and finishes to an existing structure. The building envelope is there — you’re not starting from dirt.
Basement ADU: $70,000–$130,000
Basement conversions work when you have ceiling height (at least 7 feet), egress window access, and existing structure in decent shape. In Clark County, many homes don’t have full basements — but for those that do, it’s an efficient conversion.
Attached Addition ADU: $120,000–$200,000
Building out from the side or rear of your home adds real square footage but requires foundation work, framing, and full utility connections. Fire separation from the main home adds scope. Attached additions tend to cost more than conversions but less than detached new construction.
Detached ADU (New Construction): $140,000–$280,000+
Purpose-built detached ADUs are the most expensive — and usually the most valuable. All utilities run independently, the unit has its own foundation and structure, and you’re building something that will last. In Clark County, a well-built 600–800 sq ft detached ADU at the $160,000–$220,000 range is realistic for most properties.
What Drives Cost Up in Washington Specifically
- Energy code. Washington’s residential energy code is among the strictest in the country. Insulation requirements, window efficiency, and mechanical ventilation standards add cost vs. other states.
- Site conditions. Clark County has a lot of lots with slope, clay soils, and setback constraints. Rocky or wet soils increase foundation cost. Sloped lots require more excavation.
- Utility runs. The further the ADU sits from the main house or street connections, the more it costs to get water, sewer, and electrical to it.
- Permit timeline. Clark County permitting typically runs 8–16 weeks. That’s not a cost driver directly, but it affects your overall project timeline.
What You Get for the Money
A $160,000 ADU in Clark County is typically a 500–700 sq ft detached unit with one bedroom, a full bathroom, kitchen, and living area. Finishes are builder-grade to mid-grade. It will pass all inspections, carry a full certificate of occupancy, and can be legally rented.
At current rental rates in Battle Ground and Vancouver, a well-located ADU can generate $1,400–$1,900/month. That’s a meaningful return on a $160,000 investment.
Get a Real Number for Your Property
I See Construct builds ADUs across Clark County from our base in Battle Ground. We’ve been doing this for over 30 years. A free consultation tells you what’s realistic for your lot and your budget — no pressure, no obligation.
Call (360) 989-0118 or visit iseeconstruct.com/adu-construction.