Are smoking restrictions in a Illinois lease enforceable?

Yes. Illinois landlords can prohibit smoking inside the unit, on balconies, and in common areas through a lease clause. The clause must be clear, included before lease signing, and not buried in fine print. Once signed, a tenant who violates the smoking restriction can be served a cure-or-quit notice and, if the conduct continues, evicted for breach. Marijuana smoking in legal-marijuana states is treated the same as tobacco smoking under most Illinois leases, regardless of the state's recreational or medical marijuana laws; the lease controls. The landlord generally cannot retroactively impose a smoking ban on an existing tenancy without giving the tenant notice and an opportunity to terminate the lease, but at lease renewal the restriction can be added.

Source: Illinois contract law; Illinois landlord-tenant code