Does a Illinois rent increase notice need to be in writing?

Yes. Illinois requires a written notice for any rent increase on a month-to-month tenancy or at the end of a fixed term. 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. An oral notice is generally not enforceable; the tenant can refuse to pay the increased amount and the landlord cannot evict for nonpayment of an unwritten increase. The written notice should state the current rent, the new rent, the effective date (which must be after the statutory notice period), and the landlord's signature. Delivery follows the same service rules as other landlord notices: personal delivery, substituted service, or posting plus mailing.

Source: Lease-based; 735 ILCS 5/9-207 (30-day month-to-month termination default)