Olympia, WA Accessory Structures: ADU Rules (2026)

Section 18.04.060(A) of the Olympia Municipal Code governs accessory dwelling units in the City of Olympia, Thurston County, Washington, and it applies in all residential districts. Up to two ADUs are allowed on each residential lot together with any principal unit. An ADU is permitted as a dwelling unit added to the principal unit, created within it, or detached from it, and each one must be oriented to maintain, to the extent practical, the privacy of residents in adjoining dwellings. The code points applicants to chapter 18.100 OMC, Design Review, and chapter 18.175 OMC, Infill and Other Residential.

Size is capped at 1,000 square feet of gross floor area for each ADU. Covered porches, patios and similar covered spaces do not count toward that figure, but they are limited to a total of 120 square feet for each ADU and cannot be enclosed. An ADU can be attached to the principal unit, to an accessory structure such as a garage or shop building, or to another ADU. In that case the ADU can reach 1,000 square feet and the accessory structure can reach 800 square feet, or more if the underlying zoning district allows it or a conditional use permit for a large garage has been approved.

ADUs carry the same lot coverage standards and minimum tree density provisions as a single-family residence in the underlying zone. The Director of Community Planning and Development, or the Director's designee, can allow a deviation so that the entirety of a single floor is used in a dwelling constructed two or more years before the application date. Where a lot holds an existing home of 1,200 square feet or less and a new home is proposed as the principal unit, the existing home can become the ADU. An ADU is sold separately from the principal unit only upon approval and completion of a condominium or unit lot subdivision.