Wash. Rev. Code § 81.04.300
The commission may regulate, restrict, and control the budgets of expenditures of public service companies subject to regulation by the commission as to rates and service. The commission may require each company to prepare a budget showing the amount of money which, in its judgment, is needed during the ensuing year for maintenance, operation, and construction, classified by accounts as prescribed by the commission, and shall within ten days of the date it is approved by the company file it with the commission for its investigation and approval or rejection. When a budget has been filed, the commission shall examine into and investigate it to determine whether the expenditures therein proposed are fair and reasonable and not contrary to public interest.
Adjustments or additions to budget expenditures may be made from time to time during the year by filing a supplementary budget with the commission for its investigation and approval or rejection.
RCW 81.04.300