Wash. Rev. Code § 7.28.280
In an action at law, for the recovery of the possession of real property, if either party claims the property as a donee of the United States, and under the act of congress approved September 27th, 1850, commonly called the "Donation law," or the acts amendatory thereof, such party, from the date of his or her settlement thereon, as provided in said act, shall be deemed to have a legal estate in fee, in such property, to continue upon condition that he or she perform the conditions required by such acts, which estate is unconditional and indefeasible after the performance of such conditions. In such action, if both plaintiff and defendant claim title to the same real property, by virtue of settlement, under such acts, such settlement and performance of the subsequent condition shall be prima facie presumed in favor of the party having or claiming under the elder certificate, or patent, as the case may be, unless it appears upon the face of such certificate or patent that the same is absolutely void. Any person in possession, by himself or herself or his or her tenant, of real property, and any private or municipal corporation in possession by itself or its tenant of any real property, or when such real property is not in the actual possession of anyone, any person or private or municipal corporation claiming title to any real property under a patent from the United States, or during his, her, or its claim of title to such real property under a patent from the United States for such real estate, may maintain a civil action against any person or persons, corporations, or associations claiming an interest in said real property or any part thereof, or any right thereto adverse to him, her, them, or it, for the purpose of determining such claim, estate, or interest; and where several persons, or private or municipal corporations are in possession of, or claim as aforesaid, separate parcels of real property, and an adverse interest is claimed or claim made in or to any such parcels, by any other person, persons, corporations, or associations, arising out of a question, conveyance, statute, grant, or other matter common to all such parcels of real estate, all or any portion of such persons or corporations so in possession, or claiming such parcel of real property may unite as plaintiffs in such suit to determine such adverse claim or interest against all persons, corporations, or associations claiming such adverse interest.
RCW 7.28.280