When it comes to landlords, knowing when to notarize leases is important. Luckily, Washington does not require notarization for most rental contracts.
In Washington state, it is best to have a written lease. Oral leases are valid, but they can be enforced only as leases from month to month. A lease for longer than month to month must be in writing. Per RCW § 59.04.010, if the lease is longer than one year, the execution of the lease must be acknowledged (i.e. notarized).
Nonetheless, according to Tenant's Union of Washington:
Fixed-term leases are rental agreements for a specific period of time. They must be in writing. One-year leases are very common. Under RCW 59.18.210 , leases over 12 months must be notarized in order to be valid. Leases also restrict the landlord from increasing the rent or changing the rules of tenancy during the fixed term. Tenants are obligated to meet the conditions of the lease for the full term or face penalties.
Leases for over two years may need to be recorded in the office of the county recorder for the county where the property is located. This is covered in Washington State Law under RCW 65.08.060.
As always speak with an attorney before preparing your rental agreement.