Wash. Rev. Code § 35.23.570
Before letting any contract for the construction of any waterworks for irrigation and domestic purposes, the mayor and council shall by ordinance or resolution adopt the plans therefor and shall fix and establish the assessment district, if the same is to be constructed at the expense of the district, and such cities and towns are authorized to charge the expense of such waterworks for irrigation and domestic purposes to all the property included within such district which is contiguous or proximate to any streets in which any main pipe or lateral pipe of such waterworks for irrigation and domestic purposes, is to be placed, and to levy special assessments upon such property to pay therefor, which assessment shall be levied in accordance with the last general assessment of the property within said district for city purposes.
RCW 35.23.570