Wash. Rev. Code § 35A.07.050
When a petition which is sufficient under the rules set forth in RCW 35A.01.040 is filed with the legislative body of a charter city, signed by registered voters of such city in number equal to not less than ten percent of the votes cast at the last general municipal election, seeking adoption by the city of the classification of charter code city, the county auditor shall file with the legislative body thereof a certificate of sufficiency of such petition. Thereupon the legislative body shall cause such proposal to be submitted to the voters at the next general municipal election if one is to be held within one hundred eighty days, or at a special election to be held for that purpose not less than ninety days nor more than one hundred and eighty days after the filing of such petition. Ballot titles for such election shall be prepared by the city attorney.
RCW 35A.07.050