50 ILCS 460/30
An assessment under a special assessment proceeding may be payable in one or more installments coming due not more than 31 years after the date of the judgment order of final confirmation of the assessment roll and report for such special assessment proceeding. The first installment for interest on such special assessment may be deferred for such period of time during which interest upon bonds payable from such special assessment has been provided for pursuant to the provisions of Section 50 of this Act. Installments of principal of a special assessment need not become due in equal amounts, but rather, if provided for in the special assessment ordinance, shall become due in the years and in the amounts as shall be provided in the special assessment ordinance. In the event that a governing body deems it advisable to refund Supplemental Act Assessment Bonds in a manner that would either increase installments or extend the number of years in which installments may be payable, the governing body may do so through a supplemental special assessment ordinance and proceeding under applicable special assessment law, provided the number of installments shall not be increased so as to become due beyond the original maximum term of 31 years provided in this Section. In the event such a refunding does not increase installments or extend the years in which payable, the governing body may refund Supplemental Act Assessment Bonds through a supplemental special assessment ordinance only.
50 ILCS 460/30