Wash. Rev. Code § 85.15.150
The board of any improvement district proceeding under this chapter shall, on or before the first day of September of each year, make an estimate of the costs reasonably anticipated to be required for the effective functioning of the district during the ensuing year and until further revenue therefor can be made available, and shall cause its chair or secretary to file the same with the board of county commissioners of the county containing the district and other benefited area. The board of county commissioners shall, on or before the first Monday in October next ensuing, certify the amount of the district's estimate, or such amount as it shall deem advisable, to the county treasurer. The amount so certified shall be applied by the regular taxing agencies against the benefit valuation of lands, buildings and improvements as shown by the then current complete roll of such properties certified to and filed with such county treasurer by the board of county commissioners. When thus levied, the amount of assessment produced thereby shall be added by the general taxing authorities to the general taxes against said lands and collected therewith as a part thereof. If unpaid, any delinquencies in such assessments shall bear interest at the same rate and in the same manner as general taxes and they shall be included in and be made a part of any general tax foreclosure proceedings, according to the provisions of law with relation to such foreclosures. As assessment collections are made, the county treasurer shall credit the same to the funds of the district.
RCW 85.15.150