Wash. Rev. Code § 83.110A.020
(1) Except as otherwise provided in subsection (3) of this section, the following rules apply:
(b) Any portion of an estate tax not apportioned pursuant to (a) of this subsection must be apportioned in accordance with any provision of a revocable trust of which the decedent was the settlor which provides for the apportionment of an estate tax. If conflicting apportionment provisions appear in two or more revocable trust instruments, the provision in the most recently dated instrument prevails. For purposes of this subsection (1)(b):
(2) Subject to subsection (3) of this section, and unless the decedent provides to the contrary, the following rules apply:
(a) If an apportionment provision provides that a person receiving an interest in property under an instrument is to be exonerated from the responsibility to pay an estate tax that would otherwise be apportioned to the interest:
[ 2012 c 97 s 1; 2005 c 332 s 3.]