35 ILCS 200/9-180
The owner of property on January 1 also shall be liable, on a proportionate basis, for the increased taxes occasioned by the construction of new or added buildings, structures or other improvements on the property from the date when 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 to December 31 of that year. The owner of the improved property shall notify the assessor, within 30 days of the issuance of an occupancy permit or within 30 days of completion of the improvements, on a form prescribed by that official, and request that the property be reassessed. The notice shall be sent by certified mail, return receipt requested and shall include the legal description of the property.
When, during the previous calendar year, any buildings, structures or other improvements on the property were destroyed and rendered uninhabitable or otherwise unfit for occupancy or for customary use by accidental means (excluding destruction resulting from the willful misconduct of the owner of such property), the owner of the property on January 1 shall be entitled, on a proportionate basis, to a diminution of assessed valuation for such period during which the improvements were uninhabitable or unfit for occupancy or for customary use. The owner of property entitled to a diminution of assessed valuation shall, on a form prescribed by the assessor, within 90 days after the destruction of any improvements or, in counties with less than 3,000,000 inhabitants within 90 days after the township or multi-township assessor has mailed the application form as required by Section 9-190, file with the assessor for the decrease of assessed valuation. Upon failure so to do within the 90 day period, no diminution of assessed valuation shall be attributable to the property.
Computations under this Section shall be on the basis of a year of 365 days.
35 ILCS 200/9-180