Wash. Rev. Code § 35.10.420
The submission of a ballot proposal to the voters of two or more contiguous cities for the consolidation of these contiguous cities may also be caused by the filing of a petition with the legislative body of each such city, signed by the voters of each city in number equal to not less than ten percent of voters who voted in the city at the last general municipal election therein, seeking consolidation of such contiguous cities. A copy of the petition shall be forwarded immediately by each city to the auditor of the county or counties within which that city is located.
The county auditor or auditors shall determine the sufficiency of the signatures in each petition within ten days of receipt of the copies and immediately notify the cities proposed to be consolidated of the sufficiency. If each of the petitions is found to have sufficient valid signatures, the auditor or auditors shall call a special election at which the question of whether such cities shall consolidate shall be submitted to the voters of each of such cities. If a general election is to be held more than ninety days but not more than one hundred eighty days after the filing of the last petition, the question shall be submitted at that election. Otherwise the question shall be submitted at a special election to be called for that purpose at the next special election date, as specified in RCW 29A.04.330, that occurs ninety or more days after the date when the last petition was filed.
If each of the petitions is found to have sufficient valid signatures, the auditor or auditors also shall notify the county legislative authority of each county in which the cities are located of the proposed consolidation.
Petitions shall conform with the requirements for form prescribed in RCW 35A.01.040, except different colored paper may be used on petitions circulated in the different cities. A legal description of the cities need not be included in the petitions.
RCW 35.10.420