Wash. Rev. Code § 35A.13.020
In council-manager code cities, eligibility for election to the council, the manner of electing councilmembers, the numbering of council positions, the terms of councilmembers, the occurrence and the filling of vacancies, the grounds for forfeiture of office, and appointment of a mayor pro tempore or deputy mayor or councilmember pro tempore shall be governed by the corresponding provisions of RCW 35A.12.030, 35A.12.040, 35A.12.050, 35A.12.060, and 35A.12.065 relating to the council of a code city organized under the mayor-council plan, except that in council-manager cities where all council positions are at large positions, the city council may, pursuant to RCW 35A.13.033, provide that the person elected to council position one shall be the council chair and shall carry out the duties prescribed by RCW 35A.13.030.
RCW 35A.13.020