Wash. Rev. Code § 7.28.070
Every person in actual, open and notorious possession of lands or tenements under claim and color of title, made in good faith, and who shall for seven successive years continue in possession, and shall also during said time pay all taxes legally assessed on such lands or tenements, shall be held and adjudged to be the legal owner of said lands or tenements, to the extent and according to the purport of his or her paper title. All persons holding under such possession, by purchase, devise or descent, before said seven years shall have expired, and who shall continue such possession and continue to pay the taxes as aforesaid, so as to complete the possession and payment of taxes for the term aforesaid, shall be entitled to the benefit of this section.
RCW 7.28.070