35 ILCS 200/9-155
On or before June 1 in each general assessment year in all counties with less than 3,000,000 inhabitants, and as soon as he or she reasonably can in each general assessment year in counties with 3,000,000 or more inhabitants, or if any such county is divided into assessment districts as provided in Sections 9-215 through 9-225, as soon as he or she reasonably can in each general assessment year in those districts, the assessor, in person or by deputy, shall actually view and determine as near as practicable the value of each property listed for taxation as of January 1 of that year, or as provided in Section 9-180, and assess the property at 33 1/3% of its fair cash value, or in accordance with Sections 10-110 through 10-140 and 10-170 through 10-200, or in accordance with a county ordinance adopted under Section 4 of Article IX of the Constitution of Illinois. The assessor or deputy shall set down, in the books furnished for that purpose the assessed valuation of properties in one column, the assessed value of improvements in another, and the total valuation in a separate column.
35 ILCS 200/9-155