Illinois Child Support Modification Guide

Illinois law requires a "substantial change in circumstances" under 750 ILCS 5/510. While not precisely defined by percentage, Illinois courts generally consider changes of 20% or more in income or other guideline factors as substantial. The change must be significant, continuing (not temporary), and typically should have persisted for at least 6 months. The key is whether the change, when run through the income shares calculation, results in a materially different support amount.

Generally, no. Illinois law provides that modifications are effective from the date the petition is filed, not retroactively. Any arrears that accrued under the previous order remain due. There is a very limited exception: if the obligor parent can prove they were unable to file due to disability or other extraordinary circumstances beyond their control, a court may consider some retroactive relief. However, this is rare and discretionary. Best practice is to file as soon as a substantial change occurs.

Illinois courts may deny modification or impute income at the previous level if you voluntarily reduced your income without good reason. Under Illinois case law, voluntary underemployment or unemployment to avoid support is not a basis for modification. However, voluntary changes made for legitimate reasons may be acceptable, such as: returning to school for career advancement, accepting a position with better long-term prospects, documented health issues, or relocation for a spouse's employment. You must prove the change was made in good faith.

Illinois adopted the Income Shares model in 2017 (effective July 1, 2017), replacing the previous percentage-of-obligor-income model. Under Income Shares, child support is calculated based on both parents' combined net incomes and allocated proportionally. The model considers: combined parental income, number of children, parenting time percentages, health insurance costs, childcare expenses, and extraordinary medical expenses. This model aims to approximate the amount parents would have spent on children if the family remained intact.

Timeline varies significantly by county. Cook County (Chicago) typically takes 9-18 months for contested cases due to heavy court dockets. Collar and downstate counties may be faster, averaging 4-8 months. An uncontested modification where both parties agree can be finalized in 60-90 days. Counties with mandatory mediation programs may resolve cases in 3-6 months if mediation is successful. Emergency situations (such as documented job loss with no income) may qualify for expedited temporary orders within 4-6 weeks.

Not necessarily. Illinois courts provide forms and self-help resources for pro se litigants. Many people successfully file modifications without an attorney, especially for straightforward income changes. However, consider hiring an attorney if: (1) the case involves self-employment or complex income, (2) there are disputes about imputed income or voluntary underemployment, (3) the other party has an attorney, (4) the case involves interpretation of the income shares guidelines, or (5) you are uncomfortable representing yourself in court. Many attorneys offer limited scope representation for specific tasks.

Under Illinois Income Shares guidelines, all forms of income are included: wages, salary, bonuses, commissions, overtime, tips, and other compensation. For variable income like bonuses or overtime, courts typically average the income over the past 12-24 months. If bonuses or overtime have been significantly reduced or eliminated and the change is permanent (not just a temporary dip), this can be grounds for modification. You must provide documentation showing the income history and explaining the change.

Remarriage alone is not a basis for child support modification in Illinois. The income of a new spouse is not considered in calculating child support obligations under the Income Shares model. However, remarriage may indirectly affect support if it results in changes to other factors, such as: (1) housing costs (if shared with new spouse), (2) health insurance availability or cost, or (3) childcare arrangements. You must show the remarriage caused a change to a factor directly considered in the guidelines calculation.