RCW 35.23.545

Proceedings attacking the validity of the consolidation of a city of the second class or the annexation of territory to a city of the second class shall be by quo warranto only, instituted by the prosecuting attorney of the county in which the city is located or by a person interested in the proceedings whose interest must clearly be shown. The quo warranto proceedings must be commenced within one year after the consolidation or annexation proceedings complained of and no error, irregularity, or defect of any kind shall be the basis for invalidating a consolidation or annexation after one year.