Can a Illinois 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 Illinois, the landlord can add a new fee with the same advance notice required for a rent increase (Illinois has no statewide advance notice specific to rent increases. 735 ILCS 5/9-207 sets 30 days as the month-to-month termination default, which landlords treat as the minimum for rent changes. No state rent cap; local rent control is preempted statewide.). 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; Chicago RLTO limits to $10 for monthly rent under $500 plus 5% over $500), 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: Lease-based; 735 ILCS 5/9-207 (30-day month-to-month termination default)