Wash. Rev. Code § 84.41.090
The department of revenue shall by rule establish appropriate statistical methods for use by assessors in adjusting the valuation of property between physical inspections. The department of revenue shall make and publish such additional rules, regulations and guides which it determines are needed to supplement materials presently published by the department of revenue for the general guidance and assistance of county assessors. Each assessor is hereby directed and required to value property in accordance with the standards established by RCW 84.40.030 and in accordance with the applicable rules, regulations and valuation manuals published by the department of revenue.
RCW 84.41.090
Construction-Severability-1975 1st ex.s. c 278: See notes following RCW 11.08.160.