Wash. Rev. Code § 48.20.102
There shall be a provision as follows:
PROOFS OF LOSS: Written proof of loss must be furnished to the insurer at its said office in case of claim for loss for which this policy provides any periodic payment contingent upon continuing loss within ninety days after the termination of the period for which the insurer is liable and in case of claim for any other loss within ninety days after the date of such loss. Failure to furnish such proof within the time required shall not invalidate nor reduce any claim if it was not reasonably possible to give proof within such time, provided such proof is furnished as soon as reasonably possible and in no event, except in the absence of legal capacity, later than one year from the time proof is otherwise required.
RCW 48.20.102