Updated September 4, 2026. Quick answer: Illinois taxes estates above a $4,000,000 exclusion, which is not indexed and not a true exemption, by applying the pre-2001 federal state death tax credit table to the entire taxable estate once that threshold is crossed, not just the amount over it. An estate of exactly $3,999,999 owes zero Illinois estate tax while one of $4,000,001 owes tax immediately, and a $5,000,000 taxable estate owes $391,600.
2026 exemption: $4,000,000 · Top rate: 16% (effective marginal rate under the pre-2001 federal state death tax credit table) · Key statute: 35 ILCS 405/2(b); 35 ILCS 405/3
Filing deadline and extensions
Illinois Form 700 must be filed with the Attorney General within nine months of the date of death, the same window as federal Form 706. A separate written extension request or Form 700-EXT can extend the filing date, but an extension of time to pay does not stop interest from accruing until payment is made.
No spousal portability
Illinois does not allow a surviving spouse to claim a deceased spouse’s unused exclusion the way federal law permits DSUE portability. Unlike the federal estate tax, Illinois does not offer portability, so when the first spouse dies, their $4 million Illinois exemption is lost forever unless their estate plan is specifically structured to preserve it. A direct read of the statutory definitions in 35 ILCS 405/2 contains no reference to portability or a deceased spouse’s unused exclusion.
The $4 million cliff, precisely
35 ILCS 405/2 defines the state tax credit as an amount equal to the full credit calculable under the Internal Revenue Code as in effect on December 31, 2001, with the exclusion amount fixed at $4,000,000 for deaths on or after January 1, 2013. An estate valued at $3,999,999 owes zero Illinois estate tax; an estate valued at $4,000,001 immediately owes approximately $15,000 to $20,000 in tax, because once an estate crosses the $4 million threshold, the entire estate is subject to tax, though a credit effectively shelters the first $4 million.
Nonresident decedents and Illinois-situs property
Illinois taxes a nonresident’s Illinois real estate and tangible personal property, with an explicit apportionment formula: the tax is the state tax credit reduced by the amount determined by multiplying the state tax credit by the percentage which the gross value of the transferred property not having an Illinois tax situs bears to the gross value of the total transferred property. For a nonresident, only real estate and tangible personal property physically located in Illinois has an Illinois tax situs.
One dollar over $4,000,000 taxes the whole estate, not the excess.
An estate of $3,999,999 owes nothing and one of $4,000,001 owes tax computed on the entire taxable estate. At $5,000,000 that is $391,600. When a cliff sits that close to your number, how the estate is valued and how assets are titled stops being a formality.
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The math on a $5,000,000 estate
Tax owed: $391,600. Adjusted taxable estate equals $5,000,000 minus $60,000 equals $4,940,000, per the pre-2001 formula Illinois incorporates by reference. That falls in the $4,040,000 to $5,040,000 bracket of the pre-2001 federal state death tax credit table, where the base credit is $290,800 plus 11.2% of the amount over $4,040,000. Excess over the bracket floor equals $900,000; 11.2% of $900,000 equals $100,800. Total tax equals $290,800 plus $100,800 equals $391,600. Naive comparison, taxing only the $1,000,000 excess over the exclusion at the top 16% rate, would suggest $160,000, which is $231,600 less than the actual figure, because the credit table applies to the full adjusted taxable estate, not just the excess.
Honest gaps
The Illinois Attorney General's own PDF fact sheet and Form 700-EXT could not be machine-read for verbatim quotes this session, so statutory text was confirmed via FindLaw's reproduction of the Illinois Compiled Statutes. The exact mechanism married couples use in place of portability, a state-specific QTIP-style election referenced in secondary sources, was not independently verified against primary statutory text this session.
Source note. Read from https://www.illinoisattorneygeneral.gov/Page-Attachments/form700.pdf; https://www.illinoisestatelaw.com/learning-center/illinois-estate-tax/; https://codes.findlaw.com/il/chapter-35-revenue/il-st-sect-35-405-2/; https://codes.findlaw.com/il/chapter-35-revenue/il-st-sect-35-405-3/ on 2026-09-04.
Related: estate tax by state · who actually pays the estate tax · the lifetime gift and estate tax exemption.
Statutory text read at each state's own department of revenue or legislature. General information, not legal or tax advice; exemptions, rates and filing rules change, and a qualified estate or tax professional should confirm the current figures before you rely on them.