Wash. Rev. Code § 35.10.480
If the voters of each of the cities proposed to consolidate approve the consolidation, elections to nominate and elect the elected officials of the consolidated city shall be held at times specified in RCW 35A.02.050. If the joint resolution or the petitions prescribe that councilmembers of the consolidated city shall be elected from wards, then the councilmembers shall be elected from wards under RCW 35A.12.180. Terms shall be established as if the city is initially incorporating.
The newly elected officials shall take office immediately upon their qualification. The effective date of the consolidation shall be when a majority of the newly elected members of the legislative body assume office. The clerk of the newly consolidated city shall transmit a duly certified copy of an abstract of the votes to authorize the consolidation and of the election of the newly elected city officials to the secretary of state and the office of financial management.
RCW 35.10.480