Wash. Rev. Code § 80.16.020
Every public service company shall file with the commission a verified copy, or a verified summary if unwritten, of a contract or arrangement providing for the furnishing of management, supervisory[,] construction, engineering, accounting, legal, financial, or similar services, or any contract or arrangement for the purchase, sale, lease, or exchange of any property, right, or thing, or for the furnishing of any service, property, right, or thing, other than those enumerated in this section, hereafter made or entered into between a public service company and any affiliated interest as defined in this chapter, including open account advances from or to the affiliated interests. The filing must be made prior to the effective date of the contract or arrangement. Modifications or amendments to the contracts or arrangements with affiliated interests must be filed with the commission prior to the effective date of the modification or amendment. Any time after receipt of the contract or arrangement, the commission may institute an investigation and disapprove the contract, arrangement, modification, or amendment thereto if the commission finds the public service company has failed to prove that it is reasonable and consistent with the public interest. The commission may disapprove any such contract or arrangement if satisfactory proof is not submitted to the commission of the cost to the affiliated interest of rendering the services or of furnishing the property or service described in this section.
RCW 80.16.020