Wash. Rev. Code § 35A.10.030
Upon the passage of a resolution of the legislative body of a charter code city, or upon the filing with the county auditor of a sufficient petition signed by registered voters of a charter code city in number equal to not less than ten percent of the votes cast at the last general municipal election therein, proposing abandonment of the classification of charter code city and that the city be governed under its charter and the general law relating to cities of the classification named in the petition or resolution, the legislative body thereof shall cause the propositions to 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 passage of the resolution or the filing of the certificate of sufficiency of the petition. Notice of election shall be given, the election conducted, and results declared generally as provided in chapter 35A.02 RCW, insofar as such provisions are applicable. If a majority of the votes cast upon such proposition are in favor of abandonment of the classification of charter code city, upon the certification of the record of election to the office of the secretary of state, such charter city shall be classified as a city of the class selected and shall be governed by the laws relating thereto.
RCW 35A.10.030
Sufficiency of petition in code city: RCW 35A.01.040.