Are Registered Domestic Partners Treated the Same As Spouses In Probate?
Mostly, yes. In creating Washington's legal status called "registered domestic partner," the Washington legislature intended to change all of Washington law so that "state registered domestic partners shall be treated the same as married spouses. Any privilege, immunity, right, benefit, or responsiblity 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 partnershp, related in a specified way to another individual." This intent is to 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. See also RCW 11.68.900.
Regardless of the Washington legislature's intent, some assets (IRAs, pensions, 401Ks, for example) are governed by federal law, which pre-empts Washington statutes, and so with regard to these assets, a registered domestic partner might find himself being treated differently than Washington marital partners. Washington resists this federal pressure. See, for example, RCW 11.66.900.
This is a developing area of law at present.