35 ILCS 200/9-135
If the assessor or chief county assessment officer finds that any property subject to taxation, or special assessment, has not been returned to him or her by the clerk, or has not been described in the subdivisions or manner required by this Code, he or she shall correct the return of the clerk, and shall list and assess the property in the manner required by law.
The assessor or chief county assessment officer shall, also, from time to time, make alterations in the description of property as he or she may find necessary. When property has been subdivided since the making of the general assessment, the assessor or chief county assessment officer shall from time to time correct the descriptions so that they correspond to the subdivision, and distribute the assessment in the proper proportions among the parcels into which the land has been subdivided; and in case of a vacation of a subdivision readjust the description of the assessment accordingly.
35 ILCS 200/9-135