Wash. Rev. Code § 35.10.410
The submission of a ballot proposal to the voters of two or more contiguous cities for the consolidation of these contiguous cities may be caused by the adoption of a joint resolution, by a majority vote of each city legislative body, seeking consolidation of such contiguous cities. The joint resolution shall provide for submission of the question to the voters at the next general municipal election, if one is to be held more than ninety days but not more than one hundred eighty days after the passage of the joint resolution, or shall call for a special election to be held for that purpose at the next special election date, as specified in RCW 29A.04.330, that occurs ninety or more days after the passage of the joint resolution. The legislative bodies of the cities also shall notify the county legislative authority of each county in which the cities are located of the proposed consolidation.
RCW 35.10.410