ADUs and rules

Thurston County ADU Rules Changed in December 2025. Here Is What Applies Now

An accessory dwelling unit is a second, smaller home on the same lot as your house.

It can be attached, like an apartment over the garage. Or detached, which is what most people mean by a backyard cottage, a DADU or a mother-in-law unit. Either way it needs its own kitchen, bathroom and somewhere to sleep.

Most people build one to house family or to earn rent. The rules below decide how many you can have and how big they can be. The building side is on our ADU and DADU page.

Check the ordinance, not just the code database

Most people researching an ADU land on Municode. That is where county code lives, it is what search engines surface, and it is what most AI answers are built from.

Thurston County says on its own ADU page that a new code can take up to six months to show up in Municode. When these changes were adopted on 16 December 2025, they were not there yet.

The county tells you to read Ordinance 16575 instead.

What changed in December 2025

• Family member units are no longer permitted.
• One ADU per lot is permitted in the rural area.
• Detached ADUs are permitted in the rural area and no longer require additional land.
• Size cap of 1,000 square feet, which a transferable development right can raise to 1,200.
• A rural ADU must sit within 100 feet of the primary residence, unless there are limiting circumstances.
• It must share the primary driveway.

The detached change matters more than it sounds. The old rural rules made a detached unit hard to fit on an ordinary lot.

Now what counts is how close it sits to the house and how you reach it, not how much land you own.

Urban growth areas: up to two ADUs per lot

This is the headline. Inside a county urban growth area you can have two accessory dwelling units on one lot.

The UGAs are Olympia, Lacey, Tumwater, Grand Mound, Yelm, Rainier and Tenino.

You can arrange the two units in any of three ways:

1. One attached ADU and one detached ADU.
2. Two attached ADUs.
3. Two detached ADUs, which may be in either one building or two separate buildings.

The third option is worth pausing on. Two detached units in one building is really a duplex-shaped backyard building.

On many plots, that uses the ground better than two separate cottages.

What counts as 1,000 square feet

State law (RCW 36.70A) says size limits inside a UGA must allow 1,000 square feet of gross floor area.

Gross floor area means the living space inside. The county is clear that basements and attics count.

So a 700 square foot ground floor with a 300 square foot loft is already at the cap. If you are designing close to the limit, count the loft.

Water and septic, which is where rural projects stall

This is what most often turns a quick rural ADU into a slow one.

• Rural lots need approved water and septic.
• Inside a UGA you need approved water and septic, or a connection to water and sewer.
• Either way you need a Certificate of Water Availability, a COWA.
• And the project has to meet county code 14.48.100(B).

A second home means more bedrooms on the same septic system. If yours was sized for the house alone, it may need enlarging or replacing.

That is a big cost with nothing to show for it. Find out early, not late.

Which code applies to you

Thurston County uses a different zoning title depending on which urban growth area you are in. This one is easy to get wrong.

Now the distinction that catches people out. Being in the Olympia urban growth area is not the same as being in Olympia.

If you are in the UGA but outside the city limits, the county permits your project, not the city.

The pre-approved plans, if you are inside a city

Olympia, Lacey and Tumwater share a set of pre-approved ADU plans, each with exterior options. Lacey lists four sizes, from a 480 square foot studio to an 800 square foot two bedroom.

Use one and you are not paying for drawings from scratch. Review is quicker too, because the plan has already been through it.

What it does not cover is your site. The survey, the setbacks, the utility runs, the septic, the critical areas.

Those are still yours, and they are usually where the real cost and the real delay sit. A pre-approved plan saves you design time, not project time.

The overlays that can override all of this

Meeting the ADU rules is necessary. It is not always enough. Three other sets of rules can apply, and any one of them can change what you can build.

• The Shoreline Master Program, if you are near water. In this county a great many lots are.
• The Critical Areas Ordinance, covering wetlands, steep slopes, habitat and geological hazards.
• The Health Code, which is usually the septic question in another form.

Check these before you fall in love with a design. It is far cheaper to learn at the sketch stage that half your yard is a protected wetland buffer.

Talk to the county before you draw anything

The county runs four levels of help at the Building Development Center, and the cheapest one is free.

Most people skip it and pay for it later in redesign.

Walk-ins are Monday to Friday, 8am to 1pm, no appointment, with the last ticket at 12:30pm. That window also covers submitting permits and paying fees. Appointments run from 1:30pm to 4pm.

The paid tiers are in person and need booking. A concept review wants your application a week ahead, with early sketches, square footage and rough locations. A presubmission conference expects engineer or architect drawings.

How long a permit stays alive, and what changed in March 2026

Title 14 of county code runs two separate clocks. In March 2026 the county clarified where one ends and the other begins.

• An application expires if there is no activity on it for one year.
• An issued permit expires if construction does not begin within one year.

The gap between them used to catch people out. A project could be reviewed and marked ready to issue, then sit there because nobody collected the permit and paid.

The county has now confirmed it stays an application until you pay. Paying is what issues the permit and starts the second clock.

Reached ready to issue over a year ago? The county is setting up a process to request a one-year extension, counted from the date the permit would have been issued.

From now on, once a permit is issued you have a year to start work or ask for more time.

A sensible order to do this in

1. Work out whether you are inside a city, inside a UGA but not the city, or rural. This determines everything else.
2. Confirm how many units you are allowed. Two in the UGAs, one rural.
3. Get the septic or sewer question answered before you design anything.
4. Request the Certificate of Water Availability.
5. Check the shoreline, critical areas and health overlays.
6. Book a concept review with the county before you pay anyone to draw plans.
7. Only then decide between a pre-approved plan and a custom design.
8. Budget for the site work separately from the building.

Where to go from here

We build ADUs and detached ADUs across Olympia, Lacey, Tumwater and the rest of Thurston County, on foundations rather than wheels.

Thinking smaller than an ADU? A tiny house has its own rules, and we cover them in tiny homes on a foundation or on wheels.

Tell us your address and we will tell you which rules apply to your lot, how many units you can have, and what the site is likely to demand before anything gets built.

Code requirements on this page were last checked on 7 October 2026. Codes change, so confirm current standards with your city or county before you apply.