According to RCW 84.26.030, the actual cost of substantial improvement to eligible historic property (which cost is 25% or greater of the assessed value of the historic structure prior to rehabilitation) may be excluded from the assessed value of such property for a period of ten years. Applications for this exemption are furnished by the Assessor and approved by the Historical Preservation Board of the jurisdiction within which the property is located.
Click here for the Historic Property Exemption application.