Wash. Rev. Code § 81.12.040
No public service company shall, directly or indirectly, purchase, acquire, or become the owner of any of the franchises, properties, facilities, capital stocks or bonds of any other public service company unless authorized so to do by the commission. Nothing contained in this chapter shall prevent the holding of stocks or other securities heretofore lawfully acquired or prohibit, upon the surrender or exchange of said stocks or other securities pursuant to a reorganization plan, the purchase, acquisition, taking or holding by the owner of a proportionate amount of the stocks or other securities of any new corporation organized to take over at foreclosure or other sale, the property of the corporation the stocks or securities of which have been thus surrendered or exchanged. Any contract by any public service company for the purchase, acquisition, assignment or transfer to it of any of the stocks or other securities of any other public service company, directly or indirectly, without the approval of the commission shall be void and of no effect.
RCW 81.12.040