50 ILCS 45/75
For any tax for which a taxpayer has not received a written notice of an audit, investigation, or assessment from the local tax administrator, a taxpayer is entitled to file an application with the local tax administrator for a voluntary disclosure of the tax due. A taxpayer filing a voluntary disclosure application must agree to pay the amount of tax due, along with interest of one percent per month, for all periods prior to the filing of the application but not more than 4 years before the date of filing the application. Except for the amount of tax and interest due under this Section, a taxpayer filing a valid voluntary disclosure application may not be liable for any additional tax, interest, or penalty for any period before the date the application was filed, provided, however, that if the taxpayer incorrectly determined and underpaid the amount of tax due as provided in this Section, the taxpayer is liable for the underpaid tax along with applicable interest on the underpaid tax, unless the underpayment was the result of fraud on the part of the taxpayer, in which case the application shall be deemed invalid and void. The payment of tax and interest required under this Section must be made within 90 days after the filing of the voluntary disclosure application or the date agreed to by the local tax administrator, whichever is longer, except that any additional amounts owed as a result of an underpayment of tax and interest previously paid under this Section must be paid within 90 days after a final determination and the exhaustion of all appeals of the additional amount owed or the date agreed to by the local tax administrator, whichever is longer.
50 ILCS 45/75