Wash. Rev. Code § 36.70.070
Whenever a commission is created by a county, it shall consist of five, seven, or nine members as may be provided by ordinance: PROVIDED, That where a commission, on June 10, 1959, is operating with more than nine members, no further appointments shall be made to fill vacancies for whatever cause until the membership of the commission is reduced to five, seven or nine, whichever is the number specified by the county ordinance under this chapter. Departments of a county may be represented on the commission by the head of such departments as are designated in the ordinance creating the commission, who shall serve in an ex officio capacity, but such ex officio members shall not exceed one of a five-member commission, two of a seven-member commission, or three of a nine-member commission. At no time shall there be more than three ex officio members serving on a commission: PROVIDED FURTHER, That in lieu of one ex officio member, only, one employee of the county other than a department head may be appointed to serve as a member of the commission.
RCW 36.70.070