ADUs and design
Garage to ADU Conversion: Cost, Steps and What Washington Allows
The short answer
Washington law requires cities to allow an existing structure, such as a detached garage, to be converted into an ADU. A garage conversion is also the cheapest kind of ADU to build.
The catch is that a garage was built for cars, not people. Insulation, heat, plumbing, windows and the floor all need work before it can become a home.
An ADU, or accessory dwelling unit, is a small second home on the same lot as a house. If you have a garage that mostly holds boxes, it might be the easiest place to put one. Here is what a garage ADU takes, what it costs, and when it makes sense.
Can you turn your garage into an ADU?
In most cities, yes. Washington's ADU law requires cities and counties planning under the Growth Management Act to allow the conversion of an existing structure, such as a detached garage, into an ADU. An attached garage can become an attached ADU too.
- In Thurston County's urban growth areas, around Olympia, Lacey and Tumwater, you can have up to two ADUs on one lot, attached or detached.
- In the rural county, you get one ADU. A detached one must sit within 100 feet of the main house, unless there are limiting circumstances, and needs approved water and septic.
- Anywhere, an ADU is not allowed where septic systems, critical areas or the land itself rule it out.
Our Thurston County ADU rules guide has the details. Your city or county will confirm what applies to your lot.
What a garage conversion costs
A garage conversion is the cheapest kind of ADU because the foundation, walls and roof are already there. One home addition cost guide's ADU calculator puts typical costs at:
| Kind of ADU | Typical cost per sq ft |
|---|---|
| Garage conversion | $170 to $320 |
| Basement conversion | $191 to $360 |
| Attached ADU | $234 to $440 |
| Detached ADU | $276 to $520 |
These are national averages before permits, which the same guide puts at about $5,000 to $12,000, and before the calculator adjusts for your state. As an example, a 400 square foot garage at $170 to $320 a square foot comes to roughly $68,000 to $128,000 before permits.
The guide sizes a studio at around 400 square feet and a one-bedroom at 600 to 750, so a two-car garage is often studio-sized. If you plan to rent it, the guide puts ADU rents at roughly $1 to $3 per square foot per month, depending on the market.
The work a garage needs
A garage is a shell. Turning it into a home usually means:
- Insulation and heat. Walls, ceiling and floor need insulating to Washington's energy code, and the space needs its own heat.
- The floor. Garage slabs are cold and often slope toward the door. Many conversions add a raised, insulated floor over the slab.
- The garage door. It is replaced with a framed wall, often with windows or a door.
- Plumbing. A bathroom and kitchen need drain lines, which can mean cutting into the slab to reach the sewer or septic.
- Windows. A bedroom needs an emergency escape window, and more glass makes a small space feel bigger.
- Fire separation. If the garage is attached to the house, the wall and ceiling between them usually need to be fire rated.
Making a small space work
In a space the size of a garage, every foot counts. Put the kitchen along one wall, keep the bathroom compact, and use the old garage door opening for big windows that bring in light. For more layout ideas, see ADU design ideas.
Garage conversion or build new?
- Convert if the garage is sound, the roof is in good shape, and you can live without the parking and storage.
- Build new if the garage is in poor shape, too small for the home you want, or you need to keep it for cars.
- Build above it if you want to keep the garage. We cover that in ADU design ideas, with an apartment over the garage.
Have the structure checked first. A cracked slab, rot or a tired roof can wipe out the savings that made a conversion appealing.
Where to go from here
We design and build ADUs and DADUs across Thurston and Pierce counties, including garage conversions, with the permit handled as part of the job. For the full process, see how to build an ADU.
Send us a few photos of your garage, inside and out. We will tell you whether it is a good candidate and what it would take.
Common questions
Can I convert my garage into an ADU in Washington?
In most cities, yes. Washington's ADU law requires cities and counties planning under the Growth Management Act to allow the conversion of an existing structure, such as a detached garage, into an ADU. Septic systems, critical areas and other local limits can still rule out a particular lot, so check with your city or county.
How much does it cost to convert a garage into an ADU?
One home addition cost guide puts garage conversions at about $170 to $320 per square foot as a national average, plus roughly $5,000 to $12,000 in permits. A 400 square foot garage would come to about $68,000 to $128,000 before permits.
Is a garage conversion cheaper than building a new ADU?
Usually. The same guide puts a detached ADU at about $276 to $520 per square foot, compared with $170 to $320 for a garage conversion, because the foundation, walls and roof already exist.
Can I keep my garage and still add an ADU?
Yes. You can build an ADU above the garage or build a separate one in the yard. Both cost more than a conversion but keep the parking and storage.
Code requirements on this page were last checked on 28 October 2026. Codes change, so confirm current standards with your city or county before you apply.
Thinking about an ADU?
We handle the permit work as part of the build. Tell us the site and what you want.
Where this applies
We work in every one of these towns, and each has its own page covering the rules that apply there.