Quick answer: yes — an estate can owe zero federal estate tax and still owe real money to Illinois, and the AG's own published examples show the state bill can be steep. Get a calibrated estimate in the Illinois estate tax calculator.
Two exclusions, less than a third apart
The federal estate tax exemption sits at roughly $15,000,000 per individual for 2026. Illinois's own exclusion is $4,000,000 — less than a third of the federal line. Any estate between those two numbers owes nothing federally but can face a genuine Illinois tax bill once it crosses the state's own threshold.
The two systems don't even calculate the same way
Farm families feel this gap directly
Agricultural land in central Illinois can run $10,000 to $15,000 per acre, so a few hundred acres of farmland alone can approach or exceed $4,000,000 — before counting equipment, grain, or any other assets. That's an estate comfortably under the federal $15,000,000 line, but squarely within Illinois's own, much lower threshold.
Separate returns, separate offices
Federal Form 706 goes to the IRS. Illinois Form 700 goes to the Attorney General's office, with payment made separately to the Illinois State Treasurer — two different filings, two different recipients, on two different calculations.
A local probate attorney can review your estate — many offer a free consultation.
See the full threshold and rate breakdown for exactly why this tax needs the state's own calculator rather than a table.