Wash. Rev. Code § 84.56.370
Such person may apply to the county assessor for a certificate showing the total assessed value of the land together with all buildings, structures or improvements located thereon and the assessed value of the building, structure or improvement the tax upon which the applicant desires to pay. It shall be the duty of the county assessor to issue such certificate of segregation upon written application accompanied by an affidavit attesting to the fact of separate ownership of land and improvements. Upon presentation of such certificate of segregation to the county treasurer, that officer shall segregate the total tax in accordance therewith and accept and receipt for the payment of that proportion of total tax which is shown to be due against any building, structure or improvement upon which the applicant desires to pay.
RCW 84.56.370