Wash. Rev. Code § 54.08.050
The existence of any public utility district now or hereafter formed under chapter 1, Laws of 1931, cannot hereafter be legally questioned by any person except the state of Washington in an appropriate court action brought within six months from the date that the county election board shall have canvassed the returns of the election held on the proposition of creating such district. If the existence of a district is not challenged within the period above specified, by the filing and service of petition or complaint in the action aforesaid, the state of Washington thereafter shall be barred forever from questioning the legal existence and validity of such district by reason of any defect in the organization thereof, and the same shall be deemed duly and regularly organized under the laws of this state.
RCW 54.08.050