Does Illinois State recognize an automatic federal extension?
Illinois Department of Revenue will grant a Corporation an additional extension to file of seven months for corporations if the Internal Revenue Service grants an additional extension to the Corporation. If you do not owe any tentative tax, you are not required to file Form IL-505-B.
Illinois State will recognize an automatic federal extension, but a copy of the federal extension must be attached to the Illinois Corporation tax return when it is filed.
However, if the Corporation expects a tax liability to be due, it must use Form IL-505-B to pay any tentative tax due in order to avoid interest and penalty on tax not paid by the original due date of the return."
As with all States and "an extension of time to file your Form IL-1120 does not extend the amount of time you have to pay your Illinois tax liability."