Wash. Rev. Code § 4.12.070
Any party in a civil action pending in the superior court in a county out of whose limits a new county, in whole or in part, has been created, may file with the clerk of such superior court an affidavit setting forth that he or she is a resident of such newly created county, and that the venue of such action is transitory, or that the venue of such action is local, and that it ought properly to be tried in such newly created county; and thereupon the clerk shall make out a transcript of the proceedings already had in such action in such superior court, and certify it under the seal of the court, and transmit such transcript, together with the papers on file in his or her office connected with such action, to the clerk of the superior court of such newly created county, wherein it shall be proceeded with as in other cases.
RCW 4.12.070