Wash. Rev. Code § 17.28.255
The board of trustees shall annually determine the amount of money necessary to carry on the operations of the district and shall classify the property therein in proportion to the benefits to be derived from the operations of the district and in accordance with such classification shall apportion and assess the several lots, blocks, tracts, and parcels of land or other property within the district, which assessment shall be collected with the general taxes of the county or counties. A mosquito control district must use the assessed value applicable to forestland, farm and agricultural land, or open space land, under chapter 84.33 or 84.34 RCW, when the land has been designated as such and the assessed value is used as a component in determining the district assessment. If a district uses a fractional amount of assessed value as a component in determining the district assessment, then a fractional amount of the value applicable to forestland, farm and agricultural land, or open space land, under chapter 84.33 or 84.34 RCW, shall be used.
RCW 17.28.255