Wash. Rev. Code § 80.20.060
Expenses of a complete valuation, rate and service investigation shall not be assessed against a public service company under this chapter if such company shall have been subjected to and paid the expenses of a complete valuation, rate and service investigation during the preceding five years, unless the properties or operations of the company have materially changed or there has been a substantial change in its value for rate making purposes or in any other circumstances and conditions affecting rates and services: PROVIDED, That the provisions of this section shall not be a limitation on the frequency of assessment of costs of investigation where such investigation results from a tariff filing or tariff filings by a public service company to increase rates.
RCW 80.20.060