Yes, some residential lease agreements do need to be notarized in Washington. Leases for a duration exceeding one (1) year must be notarized in order to be valid for the entire duration of the lease (WA Rev Code § 59.18.210).
Leases for less than one year are not required to be notarized. The landlord and tenant may agree to have a shorter lease notarized if they wish, but it is optional for all leases for less than one year.
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The information for this answer was found on our Washington Residential Lease Agreement answers.