Wash. Rev. Code § 35.13.100
If a proposition relating to annexation or annexation and adoption of the comprehensive plan was submitted to the voters and such proposition was approved, the legislative body shall adopt an ordinance providing for the annexation or adopt ordinances providing for the annexation and adoption of the comprehensive plan. If a proposition for annexation or annexation and adoption of the comprehensive plan and a proposition for assumption of all or of any portion of indebtedness were both submitted, and were approved, the legislative body shall adopt an ordinance providing for the annexation or annexation and adoption of the comprehensive plan including the assumption of all or of any portion of indebtedness. If the propositions were submitted and only the annexation or annexation and adoption of the comprehensive plan proposition was approved, the legislative body may, if it deems it wise or expedient, adopt an ordinance providing for the annexation or adopt ordinances providing for the annexation and adoption of the comprehensive plan.
RCW 35.13.100