35 ILCS 200/13-5
In every county which has been declared a major disaster area by the President of the United States or the Governor of the State of Illinois, the chief county assessment officer, board of review or board of appeals shall, upon application by the property owner, make a reassessment of any taxable property in the county which was substantially damaged by the disaster. The Department shall advise with the chief county assessment officers, boards of review or boards of appeals of the several counties involved in connection with such reassessment.
In the reassessment, the value of the property shall be determined as of the date of the declaration of the county as a major disaster area. If the value of any property on that date is, by reason of the disaster, less than the prior assessment, the assessment for that year shall be arrived at by dividing by 365 the sum of the 2 products obtained (a) by multiplying the prior assessment by the number of days from January 1 of that year to the date of the declaration and (b) by multiplying the value of the property as of the date of the declaration by the number of days from the date of the declaration to December 31 of that year.
If the reassessment and computations occur prior to the adjournment of the current board of review or board of appeals, the assessment of the property shall be reduced accordingly. If the board of review or board of appeals has adjourned at the time of the declaration, the Department shall convene the board of review or board of appeals to make the reassessment of property applied for after that adjournment.
35 ILCS 200/13-5