35 ILCS 200/10-160
In counties with 3,000,000 or more inhabitants, the person liable for taxes on land used for open space purposes must file a verified application requesting the additional open space valuation with the chief county assessment officer by January 31 of each year for which that valuation is desired. For taxable years prior to 2011, in counties with less than 3,000,000 inhabitants, the person liable for taxes on land used for open space purposes must file a verified application requesting the additional open space valuation with the chief county assessment officer by January 31 of each year for which that valuation is desired. For taxable year 2011 and thereafter, in counties with less than 3,000,000 inhabitants, the person liable for taxes on land used for open space purposes must file a verified application requesting the additional open space valuation with the chief county assessment officer by June 30 of each year for which that valuation is desired. If the application is not filed by January 31 or June 30, as applicable, the taxpayer waives the right to claim that additional valuation for that year. The application shall be in the form prescribed by the Department and contain information as may reasonably be required to determine whether the applicant meets the requirements of Section 10-155. If the application shows the applicant is entitled to the valuation, the chief county assessment officer shall approve it; otherwise, the application shall be rejected.
When such an application has been filed with and approved by the chief county assessment officer, he or she shall determine the valuation of the land as otherwise permitted by law and as required under Section 10-155, and shall list those valuations separately. The county clerk, in preparing assessment books, lists and blanks under Section 9-100, shall include therein columns for indicating the approval of an application and for setting out the two separate valuations.
35 ILCS 200/10-160