We automatically grant you a six-month extension of time to file your tax return. We grant an automatic six-month extension of time to file your return.Illinois offers a 6-month extension, which moves the filing deadline from April 15 to October 15 (for calendar year filers). You'll receive an electronic acknowledgment once you. To file an Illinois tax extension, complete Form IL-505-I, enclose a check or money order, and mail both to the address provided on the form. To make an extension payment, use Form IL505I, Automatic Extension Payment for Individuals. Submit proof of IRS approval when available. In the case of automatic filing extensions, proof of timely mailing of Form 4768 to the IRS will be accepted. A tax extension gives you an additional six months to file your taxes, with the extension deadline falling on Oct. 15. I already filed my taxes.