605 ILCS 5/5-916

All impact fees collected by a unit of local government shall be refunded to the person who paid the fee or to that person's successor in interest whenever the unit of local government fails to encumber by contract impact fees collected within 5 years of the date on which such impact fees were due to be paid. Refunds shall be made in accordance with this Section provided that the person who paid the fee or that person's successor in interest files a petition with the unit of local government imposing the impact fee, seeking a refund within one year from the date that such fees were required to be encumbered by contract. All refunds made shall bear interest at a rate which is at least 70% of the Prime Commercial Rate in effect at the time of the imposition of the impact fee.