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

050 permits one accessory apartment per single-family dwelling in all single-family districts. The unit must sit under the same roof as the principal dwelling, with its own exterior entrance placed away from the main entrance, and include its own kitchen and bathroom with no more than two bedrooms. Subsection E caps the apartment at 40 percent of the principal dwelling's total floor area and, in any case, at 800 square feet. 78, and Subsection I requires the unit to connect to the principal dwelling's utilities, apart from telephone and television, without separate service accounts.

An accessory dwelling permit is required before a building permit issues. 060 separately allows one detached accessory dwelling unit per lot in single-family districts, but only on a parcel of two acres or larger. The detached unit must have between 720 and 1,600 square feet of floor space, comply with the underlying zone's setbacks, lot coverage and height limits, and can only be developed after the county grants a conditional use permit, which may require removal of the dwelling once the specified occupant no longer uses it. 050. Both ADU types were codified by Ordinance 7-2005 and amended by Ordinance No. 4-2011.