50 ILCS 45/35
Taxpayers have the right to be treated by officers, employees, and agents of the local tax administrator with courtesy, fairness, uniformity, consistency, and common sense. This Section applies to any audit of a tax imposed by a unit of local government other than a municipality having a population greater than 1,000,000, except to the extent otherwise provided in Section 8-11-2.5 of the Illinois Municipal Code. Taxpayers must be notified in writing by the local jurisdiction of a proposed audit of the taxpayer's books and records clearly identifying who will be conducting the audit. For audits being conducted by third-party providers, the local jurisdiction must provide written authorization for the third-party provider to review the books and records of the taxpayer. No contact may be made by the third-party provider until the local-jurisdiction authorization is received by the taxpayer. The notice of audit must specify the tax and time period to be audited and must detail the minimum documentation or books and records to be made available to the auditor. Audits must be held only during reasonable times of the day and, unless impracticable, at times agreed to by the taxpayer. The auditor must sign a confidentiality agreement upon request by the taxpayer. Upon the completion of the audit, the local jurisdiction must issue an audit closure report to the taxpayer with the results of the audit. An auditor who determines that there has been an overpayment of tax during the course of the audit is obligated to identify the overpayment to the taxpayer so that the taxpayer can take the necessary steps to recover the overpayment. If the overpayment is the result of the application of some or all of the taxpayer's tax payment to an incorrect local government entity, then upon request by a unit of local government, the audit information must be given to any unit of local government that may be affected by an overpayment
50 ILCS 45/35