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Thurston County, WA Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in Thurston County, WA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Max ADU size: 850 square feet
ADUs per lot: One, plus main house
Detached ADU setback: 10 feet if over one story
ADU parking requirement: 1 space per unit

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Washington under recent ADU streamlining laws. The Thurston County, WA rules below cover local size, height, and setback requirements.

Summary

Thurston County caps accessory dwelling units at 850 square feet and allows only one ADU per lot with a single-family home.

These county ordinances apply to unincorporated areas of Thurston County. Incorporated cities within the county may have their own rules that take precedence over county-level regulations.

"Accessory dwelling" means a separate living unit (apartment) integrated within a single-family dwelling, or one located as a detached accessory dwelling located on the same lot as a single-family dwelling. Accessory dwellings shall be limited to eight hundred fifty square feet in floor area and, for the purposes of calculating residential density, shall not count as a dwelling unit. There shall not be more than one accessory dwelling located on a lot in addition to the single-family dwelling.

View official code

Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 77).

Full Breakdown

County code defines an accessory dwelling as a separate living unit either integrated within a single-family house or built detached on the same lot. The unit cannot exceed 850 square feet of floor area, and the code caps a lot at one accessory dwelling in addition to the primary single-family home: you cannot stack a second ADU on the same parcel. For density calculations the ADU does not count as a separate dwelling unit. In the Low Density Residential District 0-6, detached ADUs over one story need a 10-foot setback, and off-street parking for an ADU is set at one space per unit under Section 21.12.060.

Violations & Fines

Building an ADU over 850 square feet, adding a second ADU on the same lot, or skipping the building permit and required setbacks triggers a zoning code violation and stop-work/correction order from Thurston County Community Planning & Economic Development.

Frequently Asked Questions

Can I add a second ADU to my Thurston County lot?

No. County code allows only one accessory dwelling per lot in addition to the single-family home.

Does an ADU count toward my lot's density limit?

No. Section 21.06.055 says accessory dwellings do not count as a dwelling unit for residential density calculations.

Sources & Official References

[1]Thurston County Code § 21.06.055

Other rules in Thurston County

Shed & Carport RulesSome Restrictions Garage ConversionsSome Restrictions Noise Ordinances: Barking DogsSome Restrictions Parking Rules: Loading ZonesSome Restrictions Parking Rules: Driveway RulesSome Restrictions Parking Rules: Abandoned VehiclesSome Restrictions Fence Regulations: Retaining WallsSome Restrictions Animal Ordinances: Cat RulesSome Restrictions

All Thurston County rules

How Thurston County compares: Easiest Cities to Build an ADU·Compare Thurston County to another location·View the Washington accessory structures overview

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