You will owe a late-payment penalty for underpayment of estimated tax if you were required to make estimated payments and failed to do so, or failed to pay the required amount by the payment due date. You may be penalized even if you are receiving a refund. If your Illinois Income Tax exceeds the total tax withheld and credits by more than $1,000 you may have been required to make estimated payments. The late-payment penalty amount is based on the number of days the
payment is late. Payments less than 31 days
late are penalized at 2% of the amount due and payments 31+ days late are
penalized at 10% of the amount due.
For more information refer to Form IL-2210, Computation of Penalties for Individuals
and Publication 103, Penalties and Interest for Illinois Taxes