Thurston County, WA ADU (accessory dwelling unit) Rules and Regulations (2026)

Thurston County rewrote its ADU rules at the end of 2025. On December 16, 2025, the Board of County Commissioners adopted Ordinance 16575 with the Thurston 2045 comprehensive plan. The ordinance rewrote the rural ADU section, added an urban ADU section allowing two ADUs per lot, ended Family Member Units, and confirmed that rural detached ADUs need no additional land.

If you read an older summary that describes a 1,500-square-foot detached ADU, a percentage-of-house detached cap, or one ADU per parcel everywhere, it doesn’t reflect the current code. The standards below come from the county’s ADU information page and the codified text of TCC Chapter 20.34. For statewide context, see our Washington ADU laws guide.

Who reviews your lot

A Lacey or Tumwater address can be inside the city or in the county-administered UGA around it; check the parcel.

Rural Thurston County

TCC 20.34.020 applies in all Title 20 zones that allow residential uses, except the Grand Mound urban districts and the Yelm, Rainier, and Tenino UGAs.

One ADU per parcel, with a detached single-family home. An ADU can’t go on a parcel that already has any other accessory residential structure. Family Member Units are no longer permitted.

Density. Attached ADUs, conversions, and detached ADUs don’t count as dwelling units for density, and no extra land is needed beyond the parcel minimum for the house. Detached ADUs may only be permitted consistent with historical rural density patterns, as calculated by the county. Ask how that calculation applies to your parcel.

Size:

• Attached: no more than 50 percent of the principal house’s gross floor area.
• Conversion of existing interior or detached space: no size cap, as long as the conversion stays within the structure’s footprint.
• New detached: up to 1,000 square feet, or 1,200 square feet if one transferable development right is purchased from the Long-Term Agriculture district under Chapter 20.62.

Placement and design:

• A detached ADU must be within 100 feet of the primary dwelling. The director may approve a greater distance through Type I review when a well, septic system, drainfield, or critical area makes 100 feet impractical.
• An attached addition must visually match the house in type, size, trim, and materials.
• A conversion must replace any displaced parking and add parking under the county’s off-street parking standards.
• The ADU must share the house’s driveway to the public road, and the zone’s hard-surface coverage limit applies.
• The ADU may be any residential building type permitted under Title 14, Buildings and Construction.

Neither the rural nor the urban section includes an owner-occupancy requirement.

Urban growth areas

The county’s page lists up to two ADUs per lot in the Grand Mound, Yelm, Rainier, Tenino, Lacey, Tumwater, and Olympia UGAs, in any of three configurations: one attached and one detached, two attached, or two detached in one or two structures.

Grand Mound, Yelm, Rainier, and Tenino UGAs follow TCC 20.34.040. It applies to the Grand Mound R 5-12/1, R 8-24/1, and AC districts and to residential districts in the Yelm, Rainier, and Tenino UGAs:

• Two ADUs per residential parcel with any permitted principal residential structure, in any combination.
• ADUs don’t count toward density and need no extra land.
• Floor area up to 1,000 square feet.
• The zone’s design standards apply.
• ADUs may share the house’s driveway or have a secondary driveway; hard-surface coverage limits apply.

Lacey, Tumwater, and Olympia UGAs follow county Titles 21, 22, and 23. The county’s page points to state law for size: within UGAs, a county can’t set an ADU size cap below 1,000 gross square feet (RCW 36.70A.681). Check that UGA title for setbacks, height, and lot coverage.

State law also bars owner-occupancy requirements and caps ADU impact fees at half of the principal unit’s on qualifying urban lots. Counties may restrict ADUs used as short-term rentals (RCW 36.70A.680).

Water and septic

Every Thurston County ADU needs water and sanitation. For any ADU connecting to a Group A or Group B water system, the county requires a Certificate of Water Availability (COWA) and compliance with TCC 14.48.100(B)(10)–(12). The county applies that requirement even to ADUs that may not need a building permit, such as some internal conversions.

Rural ADUs need approved water and septic. UGA ADUs need approved water and septic or a sewer and water connection. The county’s page also flags the Shoreline Master Program, the Critical Areas Ordinance, and the health code as regulations that may apply.

The code’s exception to the 100-foot rule names wells, septic systems, and drainfields as limiting factors. On a rural lot, map the drainfield, its reserve area, and the well before choosing a site for a detached ADU.

How a factory-built ADU fits

Wolf homes are modular, built to Washington’s residential code — not HUD (the federal standard for manufactured homes) manufactured homes. Washington L&I reviews the factory plans and oversees factory inspections, and each home carries its L&I insignia before shipping. The county still reviews zoning, the foundation, utility connections, and site inspections.

For a rural detached ADU, test every floor plan against the 1,000-square-foot cap and the 100-foot distance. Compare our models with both. Thurston County sites are beyond the standard 20-mile delivery in Wolf’s published installation scope, so transport and setting are quoted after the site review. See cost and the Washington ADU cost guide.

What to have ready

1. Address or parcel number, and whether it is inside a city or a county UGA.
2. The zone, and any existing accessory residential structure on the parcel.
3. The septic permit and as-built, well or water-system details, and whether you already have a COWA.
4. Where the drainfield, reserve area, and well sit relative to the house.

Send us the address and we’ll confirm which Thurston County rule set applies and what fits before you pay for design.

This is general information, not legal advice. Recheck the current statute, local code, and permit packet before you file — jurisdictions amend forms and effective dates.