Wash. Rev. Code § 35A.12.010
The government of any noncharter code city or charter code city electing to adopt the mayor-council plan of government authorized by this chapter shall be vested in an elected mayor and an elected council. The council of a noncharter code city having less than twenty-five hundred inhabitants shall consist of five members; when there are twenty-five hundred or more inhabitants, the council shall consist of seven members. A city with a population of less than twenty-five hundred at the time of reclassification as an optional municipal code city may choose to maintain a seven-member council. The decision concerning the number of councilmembers shall be made by the council and be incorporated as a section of the ordinance adopting for the city the classification of noncharter code city. If the population of a city after having become a code city decreases from twenty-five hundred or more to less than twenty-five hundred, it shall continue to have a seven-member council. If, after a city has become a mayor-council code city, its population increases to twenty-five hundred or more inhabitants, the number of council offices in such city may increase from five to seven members upon the affirmative vote of a majority of the existing council to increase the number of council offices in the city. When the population of a mayor-council code city having five council offices increases to five thousand or more inhabitants, the number of council offices in the city shall increase from five to seven members. In the event of an increase in the number of council offices, the city council shall, by majority vote, pursuant to RCW 35A.12.050, appoint two persons to serve in these offices until the next municipal general election, at which election one person shall be elected for a two-year term and one person shall be elected for a four-year term. The number of inhabitants shall be determined by the most recent official state or federal census or determination by the state office of financial management. A charter adopted under the provisions of this title, incorporating the mayor-council plan of government set forth in this chapter, may provide for an uneven number of councilmembers not exceeding eleven. For the purposes of determining population under this section, cities may include or exclude the population of any state correctional facility located within the city.
A noncharter code city of less than five thousand inhabitants which has elected the mayor-council plan of government and which has seven council offices may establish a five-member council in accordance with the following procedure. At least six months prior to a municipal general election, the city council shall adopt an ordinance providing for reduction in the number of council offices to five. The ordinance shall specify which two council offices, the terms of which expire at the next general election, are to be terminated. The ordinance shall provide for the renumbering of council positions and shall also provide for a two-year extension of the term of office of a retained council office, if necessary, in order to comply with RCW 35A.12.040.
However, a noncharter code city that has retained its old mayor-council plan of government, as provided in RCW 35A.02.130, is subject to the laws applicable to that old plan of government.
RCW 35A.12.010
Severability-1979 ex.s. c 18: See note following RCW 35A.01.070.
Population determinations, office of financial management: Chapter 43.62 RCW.