Wash. Admin. Code § 458-50-090
The department shall use either the summation method or "unitary" or "enterprise" method in valuing the operating property of companies. As a general rule, the unitary or enterprise method is preferred where valuing a thoroughly integrated group of properties such that removal or destruction of any one property would jeopardize and/or immobilize the entire operation of the company. The summation method is preferred where adequate information is not available to derive reliable indicators of unitary or enterprise value, and the nature of the operating property is such that it may be segregated into component parts and the value of the parts readily determined. Notwithstanding the provisions of WAC 458-50-080, the department may, in using the summation method, employ the comparable sales or "market" approach to value to the exclusion of any other approach.
Wash. Admin. Code § 458-50-090
Order PT 75-2, § 458-50-090, filed 3/19/75.