Wash. Rev. Code § 7.28.110
A defendant who is in actual possession may, for answer, plead that he or she is in possession only as a tenant of another, naming him or her and his or her place of residence, and thereupon the landlord, if he or she applies therefor, shall be made defendant in place of the tenant, and the action shall proceed in all respects as if originally commenced against him or her. If the landlord does not apply to be made defendant within the time the tenant is allowed to answer, thereafter he or she shall not be allowed to, but he or she shall be made defendant if the plaintiff require it. If the landlord be made defendant on motion of the plaintiff he or she shall be required to appear and answer within ten days from notice of the pendency of the action and the order making him or her defendant, or such further notice as the court or judge thereof may prescribe.
RCW 7.28.110