RCW 84.12.270

The department of revenue must annually make an assessment of the operating property of all companies. Between the fifteenth day of March and the first day of July of each year the department must prepare an initial assessment roll upon which the department must enter and assess the true and fair value of all the operating property of each of such companies as of the first day of January of the year in which the assessment is made. The department must finalize the assessment roll by the twentieth day of August of each year. For the purpose of determining the true and fair value of such property the department of revenue may inspect the property belonging to the companies and may take into consideration any information or knowledge obtained by the department from an examination and inspection of such property, or of the books, records, and accounts of such companies, the statements filed as required by this chapter, the reports, statements, or returns of such companies filed in the office of any board, office, or commission of this state or any county thereof, the earnings and earning power of such companies, the franchises owned or used by such companies, the true and fair valuation of any and all property of such companies, whether operating or nonoperating property, and whether situated within or outside the state, and any other facts, evidence, or information that may be obtainable bearing upon the value of the operating property. However, in no event may any statement or report required from any company by this chapter be conclusive upon the department of revenue in determining the amount, character, and true and fair value of the operating property of such company.