Wash. Rev. Code § 35A.16.010
Upon the filing of a petition which is sufficient as determined by RCW 35A.01.040 praying for the exclusion from the boundaries of a code city of an area described by metes and bounds or by reference to a recorded plat or government survey, signed by qualified voters of the city in number equal to not less than ten percent of the number of votes cast at the last general municipal election, the legislative body of the code city shall cause the question to be submitted to the voters. As an alternate method, such a proposal for exclusion from the code city of a described area may be submitted to the voters by resolution of the legislative body. The question shall be submitted at the next general municipal election if one is to be held within one hundred and eighty days or at a special election called for that purpose not less than ninety days nor more than one hundred and eighty days after the certification of sufficiency of the petition or the passage of the resolution. The petition or resolution shall set out and describe the territory to be excluded from the code city, together with the boundaries of the code city as it will exist after such change is made.
RCW 35A.16.010