Wash. Rev. Code § 36.94.300
Whenever any land against which there has been levied any special assessment by a county shall have been sold in part or subdivided, the board of county commissioners of such county shall have the power to order a segregation of the assessment.
Any person desiring to have such a special assessment against a tract of land segregated to apply to smaller parts thereof shall apply to the board of county commissioners which levied the assessment. If the board determines that a segregation should be made, they shall by resolution order the county treasurer to make segregation on the original assessment roll as directed in the resolution. The segregation shall be made as nearly as possible on the same basis as the original assessment was levied, and the total of the segregated parts of the assessment shall equal the assessment before segregation. The resolution shall describe the original tract, the amount and date of the original assessment, and shall define the boundaries of the divided parts and the amount of the assessment chargeable to each part. A certified copy of the resolution shall be delivered to the county treasurer who shall proceed to make the segregation ordered upon being tendered a fee of three dollars for each tract of land for which a segregation is to be made. In addition to such charge the board of county commissioners may require as a condition to the order of segregation that the person seeking it pay the county the reasonable engineering and clerical costs incident to making the segregation.
RCW 36.94.300