Wash. Rev. Code § 84.40.080
An assessor shall enter on the assessment roll in any year any property shown to have been omitted from the assessment roll of any preceding year, at the value for the preceding year, or if not then valued, at such value as the assessor shall determine for the preceding year, and such value shall be stated separately from the value of any other year. Where improvements have not been valued and assessed as a part of the real estate upon which the same may be located, as evidenced by the assessment rolls, they may be separately valued and assessed as omitted property under this section. No such assessment shall be made in any case where a bona fide purchaser, encumbrancer, or contract buyer has acquired any interest in said property prior to the time such improvements are assessed. When such an omitted assessment is made, the taxes levied thereon may be paid within one year of the due date of the taxes for the year in which the assessment is made without penalty or interest. In the assessment of personal property, the assessor shall assess the omitted value not reported by the taxpayer as evidenced by an inspection of either the property or the books and records of said taxpayer by the assessor.
RCW 84.40.080