(1) Each "class of eligible historic property" shall be defined by the local legislative authority using objective criteria which lead to consistent decisions on determinations of eligibility.

(2) The criteria for the class of eligible historic property may take into account such factors as geographic location, date of construction, type of use, local landmarks designation, and other verifiable criteria consistent with the purposes of chapter 84.26 RCW, and these rules.

(3) Within historic districts, only certified historic structures may be included in the class of eligible historic property.

(4) Once a local jurisdiction has established a class of eligible historic property, it may amend the criteria defining the class at any time. However, if the new criteria are more restrictive than the previous criteria, the new criteria may not take effect for a period of two years following October 1 of the year in which the change is made. Amendments to the criteria shall not have the effect of disqualifying property already subject to special valuation.