Wash. Rev. Code § 35.13.095
A city or town may cause a proposition authorizing an area to be annexed to the city or town to be submitted to the qualified voters of the area proposed to be annexed in the same ballot proposition as the question to authorize an assumption of indebtedness. If the measures are combined, the annexation and the assumption of indebtedness shall be authorized only if the proposition is approved by at least three-fifths of the voters of the area proposed to be annexed voting on the proposition, and the number of persons voting on the proposition constitutes not less than forty percent of the total number of votes cast in the area at the last preceding general election.
However, the city or town council may adopt a resolution accepting the annexation, without the assumption of indebtedness, where the combined ballot proposition is approved by a simple majority vote of the voters voting on the proposition.
RCW 35.13.095