You must file Form IL-505-I if all of the following apply to you:
you cannot file your tax return by the due date, and
you will owe a tentative tax (complete the Tax Payment Worksheet on the back of this page to determine this), and
you submit your payment by paper check or money order (i.e., you do not use any electronic means, such as electronic funds transfer, to make your payment).
If Line 7 of the worksheet shows you owe tentative tax, you must pay the full amount due by the original due date of the return. An extension of time to file does not extend the amount of time you have to make your payment.
Do not file Form IL-505-I if:
after you complete the Tax Payment Worksheet, you find that you do not owe additional tax, or
you make your extension payment electronically by credit card, electronic funds transfer (EFT), or bank account debit.
If an unpaid liability is disclosed when you file your return, you may owe penalty and interest charges in addition to the tax. See “Will I owe penalties and interest?”
What are the extensions?
Automatic Illinois six-month extension
We automatically grant you a six-month extension of time to file your tax return. For calendar year filers, the filing date is automatically extended to October 15. You are not required to file Form IL-505-I to obtain this automatic extension. However, you must pay any tentative tax due by the original due date of your return in order to avoid any late-payment penalty and interest on unpaid tax.