Wash. Rev. Code § 26.26A.440
(1) A proceeding to establish parentage of a child under this section may be commenced only by an individual who:
(2) An individual who claims to be a de facto parent of a child must commence a proceeding to establish parentage of a child under this section:
(3) The following rules govern standing of an individual who claims to be a de facto parent of a child to maintain a proceeding under this section:
(4) In a proceeding to adjudicate parentage of an individual who claims to be a de facto parent of the child, the court shall adjudicate the individual who claims to be a de facto parent to be a parent of the child if the individual demonstrates by a preponderance of the evidence that:
[ 2018 c 6 s 509.]