Wash. Rev. Code § 35.44.410
Whenever any land against which there has been levied any special assessment by any city or town shall have been sold in part or subdivided, the legislative authority of that city or town 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 city or town which levied the assessment. If the legislative authority thereof determines that a segregation should be made, it shall by resolution order the city or town 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 city or town treasurer who shall proceed to make the segregation ordered upon being tendered a fee of ten dollars for each tract of land for which a segregation is to be made. In addition to such charge the legislative authority of the city or town may require as a condition to the order of segregation that the person seeking it pay the city or town the reasonable engineering and clerical costs incident to making the segregation. No segregation need be made if the legislative authority of the city or town shall find that by such segregation the security of the lien for such assessment will be so jeopardized as to reduce the security for any outstanding local improvement district obligations payable from such assessment.
RCW 35.44.410