Illinois Filing Due Date: Corporate income tax returns are due by March 15 — or by the 15th day of the 3rd month following the close of the taxable year (for fiscal year filers). Partnership tax returns are due by April 15 — or by the 15th day of the 4th month following the close of the taxable year (for fiscal year filers).
Extended Deadline with Illinois Tax Extension: Illinois offers a 7-month extension for filing the corporate tax return, which moves the filing deadline to October 15 (for calendar year taxpayers). Partnerships can obtain a 6-month Illinois extension, which moves the filing deadline to October 15(for calendar year taxpayers).
Illinois Tax Extension Form: Illinois business extensions are automatic, which means there is no application or written request to submit.
Illinois Extension Payment Requirement: An extension of time to file is not an extension of time to pay. If you owe Illinois tax, your balance due must be paid by the original deadline, or penalties and interest will apply. To make a state extension payment, use Form IL-505-B, or make an electronic payment via Illinois Web Pay: www.revenue.state.il.us/webpay
Illinois Tax Extension Tips: If you have a valid Federal extension (IRS Form 7004), the State of Illinois will automatically grant you an Illinois extension. In this case, you don’t have to do anything else to obtain an Illinois extension — unless you owe state tax.
You are not required to file Form IL-505-B if your Illinois tax balance is zero.