Wash. Rev. Code § 26.60.015
It is the intent of the legislature that for all purposes under state law, state registered domestic partners shall be treated the same as married spouses. Any privilege, immunity, right, benefit, or responsibility granted or imposed by statute, administrative or court rule, policy, common law or any other law to an individual because the individual is or was a spouse, or because the individual is or was an in-law in a specified way to another individual, is granted on equivalent terms, substantive and procedural, to an individual because the individual is or was in a state registered domestic partnership or because the individual is or was, based on a state registered domestic partnership, related in a specified way to another individual. The provisions of chapter 521, Laws of 2009 shall be liberally construed to achieve equal treatment, to the extent not in conflict with federal law, of state registered domestic partners and married spouses.
RCW 26.60.015