You have a letter from the Illinois Department of Revenue
The letter names a tax (income, sales, withholding), the periods under review, and the records the auditor wants to see. It may be a plain audit notice, or it may already be a Notice of Proposed Deficiency with a dollar figure attached. Either way, a clock has started, and the costly mistakes happen in the first two weeks: answering the auditor from memory, sending more than was asked for, or putting the envelope in a drawer.
This article explains what an Illinois Department of Revenue audit can examine, how far back the Department (IDOR) can reach, which deadlines are fixed by statute, and what to do in the first week. It is written for individuals and small businesses in Cook County. Our Illinois Department of Revenue page describes how Khatib Law LLC handles these matters; the tax attorney hub covers IRS problems as well.
What an Illinois Department of Revenue audit can examine
IDOR administers the Illinois Income Tax Act, the Retailers' Occupation Tax Act (the sales tax) and the other state tax statutes. Each of those Acts gives it the power to look behind a return.
- Your books and records. Every record the Income Tax Act requires you to keep is "subject to inspection by the Department or its duly authorized agents and employees" during business hours (35 ILCS 5/913). The sales tax statute reads the same way. In practice the auditor asks for general ledgers, bank statements, sales journals, exemption certificates, Forms 1099 and W-2, and the federal return.
- Correcting a return on its own information. Under the Retailers' Occupation Tax Act, IDOR examines each sales tax return and, if necessary, corrects it using the information it has (35 ILCS 120/4). A return corrected by the Department is "prima facie correct," which shifts the burden to you to prove the figure wrong with records. If you cannot document a sale as exempt, the auditor treats it as taxable.
- Federal tax information. Federal law lets the IRS share returns and return information with state tax agencies for state tax administration (26 U.S.C. 6103(d)). IDOR uses it, and many income tax audits begin with a mismatch between the federal figures and the Illinois figures. If the IRS adjusts your federal return, you are required to report the change to Illinois yourself: an amended Illinois return is due within 120 days after the federal change becomes final (35 ILCS 5/506(b)).
- Hearings. IDOR runs its own administrative hearings and can compel records and testimony. Larger disputes go to the Illinois Independent Tax Tribunal, and both routes end in the circuit court if you keep appealing.
How far back can IDOR go?
For Illinois income tax, the limits are set by 35 ILCS 5/905:
| situation | deadline for a notice of deficiency |
|---|---|
| ordinary return | 3 years after the return was filed |
| more than 25% of base income left off the return | 6 years after filing |
| no return filed, or a fraudulent return | no time limit |
| federal change reported to IDOR under 506(b) | 2 years after you report it, limited to that change |
| federal change never reported | no time limit for that year |
| written extension signed with IDOR | the date you agreed to |
Sales tax runs on half-year cycles. A notice of tax liability issued on a January 1 or July 1 generally cannot reach receipts from more than three years before that date, unless the return was fraudulent or you signed an extension (35 ILCS 120/4).
Two practical points follow. Keep at least four full years of records, and keep them longer for any year that is still open. And do not sign a statute-extension form without advice. Auditors ask for one when they are running out of time, and the scope of the extension is negotiable.
The notices, in order
An IDOR matter moves through fixed stages, and each stage has its own deadline. IDOR's publication PIO-60 and its "options to dispute" page describe the sequence.
- The audit letter. It states the tax type, the periods, and the records to produce, and it names the auditor. Appoint a representative with Form IL-2848, Power of Attorney, so the auditor deals with your representative rather than with you.
- Notice of Proposed Deficiency or Proposed Liability. These are the auditor's findings. You have 60 days to ask IDOR's Informal Conference Board (ICB) to review them. The ICB is an internal review, not a hearing, and it resolves many documentation disputes.
- Notice of Deficiency (income tax) or Notice of Tax Liability (sales and other taxes). This is the protestable notice. Within 60 days of its issuance (150 days if you are outside the United States, for an income tax Notice of Deficiency only), you must either file a written protest and request an administrative hearing or file a petition with the Illinois Independent Tax Tribunal (35 ILCS 5/908; 35 ILCS 120/4). Do neither and the amount becomes a final assessment with no further review of the merits.
- Which forum. The Tax Tribunal has jurisdiction when the tax at issue in a notice, or in several notices for the same year or audit period, exceeds $15,000 exclusive of penalties and interest, or when a penalty-and-interest-only notice exceeds $15,000 (35 ILCS 1010/1-45). Smaller cases stay with IDOR's administrative hearings. Either decision can be appealed to the circuit court.
- Final liability, then collection. Once the assessment is final, IDOR can file liens, levy bank accounts and wages, and, for a business, revoke the certificate of registration it needs to make taxable sales. At that point the remaining relief is the Board of Appeals, described below, or payment.
Your rights during the audit
The Taxpayers' Bill of Rights (20 ILCS 2520/4) requires IDOR to:
- give you a written statement of your rights with every protestable notice, bill or claim denial;
- abate tax and penalties that were assessed because of erroneous written advice from the Department;
- hold seized property and seized bank accounts in escrow for 20 days so that a Department error can be corrected;
- let you record audit interviews, under rules that set the time and place;
- pay interest on your overpayments at the same rate it charges on underpayments;
- send a closing letter if the audit finds no changes, and give you the audit findings and methods in writing if it does, along with written guidance on minimum record-keeping if you ask for it.
You also have the right to be represented by the person you choose. Nothing requires you to meet the auditor without your representative in the room.
What to do in the first week
- Read the notice twice and calendar every date. Note the tax type, the periods, the auditor's name, and the response date.
- Do not call the auditor to explain. Statements made on the phone become part of the file. Early conversations should go through your representative.
- Pull the federal and Illinois returns for every period named and reconcile them line by line. The difference between the two is usually where the audit started.
- Gather the records in the order the notice lists them. Produce copies, not originals, and produce what was requested rather than everything you have.
- Check whether the IRS changed anything. If a federal adjustment exists and was never reported to Illinois, the 120-day rule was already missed and the strategy changes before the first meeting. If the IRS is also collecting, our article on IRS collection notices explains which federal letter means what.
- File Form IL-2848. Until a power of attorney is on file, IDOR will keep writing to you.
- Do not pay a proposed figure to make it go away before someone has checked the math, the periods, and whether the limitations period had already run. Paying does not waive a refund claim, but a refund claim is a slower road than a protest.
After the liability is final: the Board of Appeals and paying under protest
If the protest deadline has passed, two doors remain.
The Board of Appeals (petition on Form BOA-1) can waive penalties and interest for reasonable cause, and it can accept an offer in compromise when it is unlikely the full debt can ever be collected. It cannot redetermine the tax itself, and it acts only after the liability is final. The nearest federal equivalent is first-time penalty abatement, which the IRS grants on request rather than by petition.
Payment under protest (Form RR-374) lets a taxpayer who wants a court rather than an administrative hearing pay the amount, then sue in the circuit court for its return; the injunction must be served within 30 days of payment. It is a narrow path, used mostly in larger business cases, and it requires money up front.
If the problem is the IRS rather than Illinois, see our pages on IRS audit representation and tax debt relief. Business owners with sales tax or withholding exposure should read the business tax attorney page, and new businesses registering through MyTax Illinois will find the registration steps in our Illinois LLC guide.
Why an attorney who is also a CPA
An IDOR audit is an accounting exercise with a legal deadline attached. The auditor wants reconciled ledgers; the protest needs a statute, a limitations argument and, sometimes, a hearing. Hani H. Khatib, Attorney at Law · CPA · LL.M. (Taxation), handles both sides in one engagement. Legal advice about how to answer the auditor and whether to protest is protected by attorney-client privilege. The privilege does not cover the work of preparing the return itself, even when a lawyer prepared it, so the first thing we explain is what is and is not protected. And if the matter is not resolved on paper, an attorney can take it to the Tax Tribunal or the circuit court. More on how the two licenses work together is on our attorney-CPA page.
Frequently asked questions
How long does an Illinois tax audit take?
A correspondence audit of one issue can close in a few months. A field audit of a business's sales tax for three years commonly runs six months to a year, longer if records are incomplete. Responding completely and on time is the single thing that shortens it.
Can IDOR audit me if the IRS already did?
Yes. A federal audit does not close the Illinois years, and IDOR receives federal adjustments through its information-sharing agreement with the IRS. You are required to report a final federal change on an amended Illinois return within 120 days. If you do, IDOR has two years to assess the Illinois effect of that change; if you do not, that year stays open indefinitely.
What happens if I cannot find the records?
Reconstruct them. Bank and credit card statements can be re-ordered, vendors can reissue invoices, and the IRS can supply wage and income transcripts. Where records are truly gone, the auditor may estimate, and the estimate is treated as correct unless you can rebut it. Say so early rather than letting the deadline pass.
Can I set up a payment plan with IDOR?
Yes, after the amount is agreed or final. IDOR offers installment payment plans through MyTax Illinois, and the Taxpayers' Bill of Rights bars IDOR from cancelling a plan while you provide accurate financial information, pay on time, and answer its requests. A payment plan does not stop penalties and interest from accruing.
What is the Informal Conference Board?
An internal IDOR review you can request within 60 days after a Notice of Proposed Deficiency or Proposed Liability. A reviewer who was not the auditor looks at your documents and arguments. It is faster and cheaper than a hearing, and it does not waive your later right to protest.
General information as of March 15, 2025, revised October 4, 2026; not legal advice; laws change; consult a lawyer about your situation.