Wash. Rev. Code § 59.16.030
It shall not be necessary for the plaintiff, in proceedings under this chapter, to allege or prove that the said lands were, at any time, actually occupied prior to the defendant's entry thereupon, but it shall be sufficient to allege that he or she is the legal owner and entitled to the immediate possession thereof: PROVIDED, That if the defendant shall, by his or her answer, deny such ownership and shall state facts showing that he or she has a lawful claim to the possession thereof, the cause shall thereupon be entered for trial upon the docket of the court in all respects as if the action were brought under the provisions of *chapter XLVI of the code of eighteen hundred and eighty-one.
RCW 59.16.030
*Reviser's note: "chapter XLVI of the code of eighteen hundred and eighty-one" is codified as RCW 7.28.010, 7.28.110 through 7.28.150, and 7.28.190 through 7.28.270.