Can a Washington landlord introduce a new fee mid-tenancy?

Generally no, on a fixed-term lease. The lease locks the rent and any specified fees for the term; introducing a new fee (parking, pet, trash, key replacement) without tenant agreement is a unilateral lease modification, not enforceable. On a month-to-month tenancy in Washington, the landlord can add a new fee with the same advance notice required for a rent increase (Washington HB 1217 (2024) requires 90 days advance notice for any rent increase, prohibits any increase during the first 12 months, and caps annual increases at the lower of CPI + 7% or 10% flat. Buildings under 12 years old are exempt. The rent cap framework sunsets July 1, 2040.). Once the notice period passes, the tenant either accepts the new fee by continuing to pay or terminates the tenancy. Some fees are statutorily controlled regardless of the lease: late fees (no statewide cap), application fees, and fees that exceed the actual cost of the service. A "new" fee that effectively re-categorizes a previously-bundled cost is treated as a rent increase.

Source: RCW 59.18.140 (90-day advance notice for any rent increase, HB 1217)