Costs

How Much Does an ADU Cost in Washington? Olympia, Lacey and Tacoma (2026)

Maybe a parent needs to move closer. Maybe you want rental income, or a place for a grown kid to land. Either way, the first question about an ADU is always what it will cost.

Here is what an ADU costs in Washington in 2026, why two quotes can be $100,000 apart, and what is different in Olympia, Lacey, Tumwater and Tacoma. For the process itself, see how to build an ADU.

ADU cost by type

The kind of ADU you build matters more than anything else. Converting space that already has walls and a roof is far cheaper than starting from bare ground.

These are national averages from homeadditioncostcalculator.com's ADU calculator, from economy to premium finishes, before permits. The calculator then adjusts them for each state's building costs, so treat them as a starting point, not a quote.

ADU cost by size

Here is what those rates mean for common sizes. Tacoma's pre-approved plans start at 300 square feet, and state law does not let cities cap an ADU below 1,000.

Permits come on top. Small ADUs tend to cost more per square foot than big ones, because a kitchen, a bathroom and the utility hookups cost about the same whatever the size.

The site work the per-foot price leaves out

This is where ADU budgets go wrong. The calculator's own figures are "before permits and utilities", and the site work can be a large share of the job.

• Utility lines. A detached ADU sits away from the house, so sewer, water and power have to be trenched out to it. Tacoma says each detached unit needs its own outside connections to sewer and water.
• Septic. In rural Thurston County an ADU needs approved water and septic and a Certificate of Water Availability before it can be permitted. If your system cannot take a second home, price that first.
• Electrical. An older house may need a bigger panel. A Washington cost guide puts panel work at about $1,500 to $4,000.
• Slopes and fill. A geotechnical report runs about $1,000 to $3,000, and the engineering it triggers can cost more.
• Impact fees. State law caps impact fees on an ADU at half of what the main house would be charged.
• A contingency of 15 to 20 percent on top of the estimate.

Olympia, Lacey and Tumwater

State law requires cities in urban growth areas to allow two ADUs per lot, to allow them up to at least 1,000 square feet, and not to require the owner to live on site. Some local codes are still catching up, so the exact rules depend on your address.

Lacey offers four pre-approved detached ADU designs, from one to two bedrooms. The city says they eliminate design costs, though you are free to use your own plans.

In rural Thurston County you can have one detached ADU per lot, up to 1,000 square feet and within 100 feet of the main house. Our Thurston County ADU rules guide has the details for each city.

Tacoma

Tacoma caps an ADU at 1,000 square feet of living space and requires no parking for it. A detached ADU goes in the rear yard, at least 5 feet from the side property lines and 15 feet from the rear, with no rear setback needed from an alley.

The city also offers pre-approved detached ADU plans, from a 300 square foot studio and a 520 square foot one-bedroom to an 845 square foot two-bedroom over a garage. Tacoma says preparing the plans and evaluating the site for services is included and initially free, which takes a real line item off the budget.

Permits depend on the job and can include building, plumbing, mechanical, side sewer, stormwater, water and electrical. More on building there is on our Tacoma page.

What about a prefab ADU?

A prefab or modular ADU is built in a factory and set on your lot. The advertised price is usually the unit alone. Foundation, delivery, a crane, utility hookups and permits come on top, so compare the all-in price, not the sticker price.

An approved modular building in Washington carries a gold insignia from L&I, the state agency that approves factory-built structures. Your city or county still permits the foundation and hookups.

Ways to bring the cost down

• Convert what you have. A garage conversion starts at about $170 per square foot, against $276 for a detached ADU. See our guide to garage ADU conversions.
• Use a pre-approved plan in Lacey or Tacoma to save on design.
• Build close to the house so the utility trenches stay short.
• Keep the shape simple. A plain rectangle under one roof line costs less than corners and angles.
• Go smaller, but not too small. The kitchen and bathroom cost about the same either way.

Can rent pay it back?

The same calculator puts ADU rents at about $1 to $3 per square foot per month, depending on the market. A 600 square foot ADU at $2 would bring in about $1,200 a month before costs.

If you are thinking short-term rental, check first. Tacoma allows an ADU to be a short-term rental only if the owner lives in the main house or the ADU.

Where to go from here

We design and build ADUs and DADUs across Thurston and Pierce counties, from garage conversions to backyard cottages, with the permit handled as part of the job.

Tell us about your lot and what the ADU is for. We will tell you which kind fits and what to budget for.

Code requirements on this page were last checked on 5 November 2026. Codes change, so confirm current standards with your city or county before you apply.