Can a Florida 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 Florida, the landlord can add a new fee with the same advance notice required for a rent increase (Florida has no statewide advance notice for rent increases. Fla. Stat. 83.57 (post-HB 1417, effective July 1, 2023) requires 30 days notice to terminate a month-to-month tenancy, which landlords use as the default for rent changes. No state rent cap.). 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, but late fees must be reasonable under Fla. Stat. 83), 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: Fla. Stat. 83.57 (30-day month-to-month termination default, post-HB 1417)