Wash. Rev. Code § 35A.09.020
Upon the filing with the county auditor of a sufficient petition signed by registered voters of a charter code city, in number equal to at least ten percent of the votes cast at the last general municipal election, seeking the adoption of a specified charter amendment set forth in the petition, providing for any matter within the realm of local affairs, or municipal business, or structure of municipal government, offices, and departments, said amendment shall be submitted to the voters at the next general municipal election if one is to be held within one hundred and 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 the certificate of sufficiency of the petition. The proposed charter amendment shall be published as provided in RCW 35A.09.050. Upon approval by a majority of the registered voters voting thereon, such amendment shall become a part of the charter organic law governing such charter code city.
RCW 35A.09.020
Sufficiency of petition in code city: RCW 35A.01.040.