Thurston County, WA Accessory Structures: Garage Conversions (2026)

020(4) sets different size rules for each: an attached ADU is capped at fifty percent of the principal home's gross floor area and a new detached ADU tops out at one thousand five hundred square feet or forty percent of the principal structure, whichever is less, but a conversion of existing internal or detached space, the classic garage-to-living-space project, carries no floor area limit at all. The only constraint is that the conversion stay within the four walls of the structure being converted; expanding the footprint turns the project into a detached or attached ADU subject to those tighter caps instead.

One ADU is allowed per residential parcel with a detached single-family home, and a conversion does not count as a separate dwelling unit for density purposes, so it does not require additional lot area beyond what the primary house already needs. 030's off-street parking table, and any remodeling of the converted space must meet the design standards of the underlying zoning district. 100(B)(10) through (12) before the conversion can be occupied. 030(6), and every conversion must still conform to Title 14's building code.