Wash. Rev. Code § 71.34.406
In addition to the responsibility provided for by RCW 43.20B.330, the parents of a minor person who is involuntarily detained pursuant to this chapter for the purpose of treatment and evaluation outside of a facility maintained and operated by the department shall be responsible for the cost of such care and treatment. In the event that an individual is unable to pay for such treatment or in the event payment would result in a substantial hardship upon the individual or his or her family, then the county of residence of such person shall be responsible for such costs. If it is not possible to determine the county of residence of the person, the cost shall be borne by the county where the person was originally detained. The department, or the authority, as appropriate, shall, pursuant to chapter 34.05 RCW, adopt standards as to (1) inability to pay in whole or in part, (2) a definition of substantial hardship, and (3) appropriate payment schedules. Financial responsibility with respect to services and facilities of the department shall continue to be as provided in RCW 43.20B.320 through 43.20B.360 and 43.20B.370.
RCW 71.34.406