Wash. Rev. Code § 84.56.035
A local government authorized both to impose and to collect any special assessments, excise taxes, or rates or charges may contract with the county treasurer or treasurers within which the local government is located to collect the special assessments, excise taxes, rates, or charges. If such a contract is entered into, notice of the special assessments, excise taxes, or rates or charges due may be included on the notice of property taxes due, may be included on a separate notice that is mailed with the notice of property taxes due, or may be sent separately from the notice of property taxes due. County treasurers may impose an annual fee for collecting special assessments, excise taxes, or rates or charges not to exceed one percent of the dollar value of special assessments, excise taxes, or rates or charges collected.
RCW 84.56.035