Wash. Rev. Code § 35.02.035
The county auditor shall within thirty days from the time of receiving said petition determine if the petition contains a sufficient number of valid signatures. If the proposed city or town is located in more than one county, the auditor shall immediately transmit a copy of the petition to the auditor of the other county or counties within which the proposed city or town is located. Each of these other county auditors shall certify the number of valid signatures thereon of voters residing in the county and transmit the certification to the auditor of the county with whom the petition was originally filed. This auditor shall determine if the petition contains a sufficient number of valid signatures. If the petition is certified as having sufficient valid signatures, the county auditor shall transmit said petition, accompanied by the certificate of sufficiency, to the county legislative authority or authorities of the county or counties within which the proposed city or town is located.
RCW 35.02.035