35 ILCS 200/9-160
On or before June 1 in each year other than the general assessment year, in all counties with less than 3,000,000 inhabitants, and as soon as he or she reasonably can in counties with 3,000,000 or more inhabitants, the assessor shall list and assess all property which becomes taxable and which is not upon the general assessment, and also make and return a list of all new or added buildings, structures or other improvements of any kind, the value of which had not been previously added to or included in the valuation of the property on which such improvements have been made, specifying the property on which each of the improvements has been made, the kind of improvement and the value which, in his or her opinion, has been added to the property by the improvements. The assessment shall also include or exclude, on a proportionate basis in accordance with the provisions of Section 9-180, all new or added buildings, structures or other improvements, the value of which was not included in the valuation of the property for that year, and all improvements which were destroyed or removed. In case of the destruction or injury by fire, flood, cyclone, storm or otherwise, or removal of any structures of any kind, or of the destruction of or any injury to orchard timber, ornamental trees or groves, the value of which has been included in any former valuation of the property, the assessor shall determine as near as practicable how much the value of the property has been diminished, and make return thereof.
Beginning January 1, 1996, the authority within a unit of local government that is responsible for issuing building or occupancy permits shall notify the chief county assessment officer, by December 31 of the assessment year, when a full or partial occupancy permit has been issued for a parcel of real property. The chief county assessment officer shall include in the assessment of the property for the current year the proportionate value of new or added improvements on that property from the date the occupancy permit was issued or from the date the new or added improvement was inhabitable and fit for occupancy or for intended customary use until December 31 of that year. If the chief county assessment officer has already certified the books for the year, the board of review or interim board of review shall assess the new or added improvements on a proportionate basis for the year in which the occupancy permit was issued or the new or added improvement was inhabitable and fit for occupancy or for intended customary use. The proportionate value of the new or added improvements may be assessed by the board of review or interim board of review as omitted property pursuant to Sections 9-265, 9-270, 16-50 and 16-140 in a subsequent year on a proportionate basis for the year in which the occupancy permit was issued or the new or added improvement was inhabitable and fit for occupancy or for intended customary use if it was not assessed in that year.
35 ILCS 200/9-160