How to Modify Child Support in Illinois

When Illinois allows a child support modification

Child support modification is governed by 750 ILCS 5/510. An order can be changed on one of a few grounds.

Substantial change in circumstances

The most common ground is a substantial change in circumstances (section 510(a)(1)). HFS lists examples such as a job loss, a new or added job, income changes, custody or visitation changes, changes in family size, disability, incarceration and deployment to active service. Whether a change is substantial is for the judge to decide. The fact that a change could have been foreseen is not a defense, unless the order or the parents' agreement in the order expressly names that future event. Parents can agree, or the judge can state in the order, that a specific future event will not count as a substantial change.

Section 510(a)(2) also allows a modification without a substantial change in two situations:

• The guideline amount differs from the existing order by at least 20 percent, and by at least $10 a month, unless the difference comes from an earlier deviation that has not changed. This ground applies only in cases receiving HFS child support enforcement services, and only when at least 36 months have passed since the order was entered or last modified.
• The child's health care needs must be provided for through health insurance or other means.

Changes in parenting time

A change in overnights can change support, especially near the shared physical care threshold. Through December 31, 2026, the shared care calculation applies when each parent has 146 or more overnights a year. Starting January 1, 2027, Public Act 104-0733 lowers that threshold to 110 overnights or overnight equivalents, with an adjustment for parents with 110 to 145 overnights. Section 510, as amended for 2027, says a new Public Act does not by itself count as a substantial change unless that Act expressly says so, and Public Act 104-0733 does not say so. Focus on what has actually changed in your family. If the schedule itself is changing, you may also need to modify parenting time; see our custody modification page.

Also starting January 1, 2027, section 510 adds a process for adjusting support when a parent is incarcerated for more than 180 consecutive days.

Filing a petition to modify in Cook County

If a court entered your order, you ask that court to change it. In Cook County, that usually means the Domestic Relations Division at the Daley Center, filed through the Clerk's e-filing system. The Illinois Courts statewide forms page explains that to change ordered child support, you file a motion asking the judge to do that. The statewide Motion form can ask for an increase or a decrease. Our guide to post-decree motions covers other requests after a judgment.

1. Get a copy of the current support order and the worksheet or findings behind it.
2. Estimate the new guideline amount using current incomes, overnights and the HFS schedule. Our child support tools explain their limits.
3. Prepare the motion or petition stating the change and the relief you want.
4. Complete a current Financial Affidavit (Family and Divorce Cases) with supporting documents.
5. File, get a court date and send notice to the other parent. If HFS is a party, ask how it must be notified.
6. Attend the hearing with your documents.

What to attach

• Your financial affidavit with recent pay stubs, tax returns and bank statements.
• Proof of the change, such as a termination letter, an unemployment benefits notice, medical records or a new job offer.
• A parenting calendar if overnights changed.
• Records of child care, health insurance and activity costs.
• A copy of the existing order.

Service on the other parent

The other parent must receive notice of the motion and the court date. The statewide forms include a Notice of Court Date for Motion and a Proof of Delivery that shows how and when you sent it. Keep that proof. If you cannot find the other parent, or the case has been closed for years, ask the court or a lawyer how notice must be given.

Retroactivity: why you should file promptly

This rule can cost a parent money if they wait. Section 510(a) says support may be modified only as to installments that come due after due notice of the filing of the motion. A judge generally cannot reach back to the months before the other parent received notice. If you lost your job in March but file in August, the payments due in between are generally still owed at the old amount.

The same rule applies to increases. If the paying parent's income rose, an increase generally starts only after notice of the filing. Meanwhile, keep paying the current order. A pending motion to modify does not stop enforcement of the existing order (section 510(f)). Read about enforcing a child support order.

HFS review and private petitions

If HFS Child Support Services handles your case, you can ask HFS for a review by calling 888-245-1938, using its online modification form or emailing the forms it lists. HFS says either parent or the child's legal guardian can ask, that its cases are reviewed every three years and that, as a rule, a 20 percent increase or decrease is required for a change. HFS will ask for proof of the change, and the order may go up or down.

HFS also says that even if Child Support Services denies a request, you can still go to court on your own. A private petition lets you present your own evidence and arguments to a judge, with your own lawyer if you choose. The HFS attorney does not represent either parent.

Recalculating support under the income shares guidelines

A modified amount is calculated under the same income shares guidelines in 750 ILCS 5/505:

1. Determine each parent's monthly net income: gross income minus the standardized or individualized tax amount and certain adjustments.
2. Add the two incomes to get the combined net income.
3. Find the basic obligation in the HFS schedule for that income and number of children.
4. Split the basic obligation between the parents according to each parent's percentage share of the combined income.

Then the court applies any shared care calculation and decides add-ons such as child care, extracurricular costs and health insurance. For low-income parents, there is a rebuttable presumption of a minimum order of $40 per month per child, up to $120 per month in total. The guideline amount is presumed correct, and a judge who deviates must explain why in writing. Online calculators help you estimate, but they do not change an order. Our guide to the 2026 and 2027 child support rules explains which materials apply to which dates.

What happens at the hearing

At the hearing, the parent asking for the change explains it and supports it with documents and testimony. The other parent can respond, challenge the income figures and argue that the change is not substantial or was voluntary. If a parent quit a job or cut hours on purpose, the court can base support on potential income, but only after an evidentiary hearing or by agreement (section 505(a)(3.2a) and (3.2b)). The judge applies the guidelines, considers any requested deviation and then enters a modified order or denies the request. Parents can also agree on a new amount and submit an agreed order for the judge's approval.

Common mistakes

• Waiting months to file after a job loss or raise.
• Stopping or reducing payments without a court order.
• Relying on an informal agreement that never became a court order.
• Filing an incomplete financial affidavit.
• Quitting a job or reducing hours voluntarily.
• Assuming the 2027 shared-care change applies to your order automatically.
• Forgetting health insurance and other add-on expenses.

How Chicago Family Attorneys can help

Chicago Family Attorneys, LLC handles child support modifications in Cook, DuPage, Will and Lake Counties. The firm practices only Illinois family law. Our child support lawyers can:

• Review whether your change is likely to qualify under section 510.
• Calculate guideline support with current incomes and overnights.
• Draft and file the motion or petition and the financial affidavit.
• Use discovery when the other parent's income is disputed.
• Respond to a modification request filed by the other parent.
• Handle a related parenting time modification.
• Appear at a single hearing on a limited-scope basis.

For full representation to set or modify child support, the usual fee is an initial payment of $1,500 to $2,500, then $500 to $650 per month beginning 30 days after the initial payment while representation remains active. Limited-scope drafting of a motion for you to file starts at $500, and one hearing appearance starts at $1,200. See child support lawyer fees. No lawyer can promise a specific support amount.

Related reading: do I need a lawyer for child support? and how to get child support in Illinois.