I don't think the law requires a landlord to change the locks after the last tenant. In my city (I am not in practice anywhere in Washington nor in Spokane) the landlord is required to provide a dead bolt lock on the door and ensure the windows lock for security.
Check with a local building and zoning administrator because the regulation is likely a city one.
Good luck to you.
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RCW 59 18 060 requires the landlord to provide you with "reasonably secure locks". Landlords would be remiss in their duties to not change the locks between tenants. Hope this helps. Elizabeth Powell