The Illinois property tax calendar, and why you pay a year late
Assessed one year, billed the next. Everything about timing follows from that.
Illinois property tax runs a year in arrears, which means at any moment three different years are in play. Getting them confused is why people appeal the wrong year.
The three years
| Year | What happens |
|---|---|
| Assessment year | Property valued as at 1 January. Roll published by township. Appeals filed. |
| Following year | Rates set, bills issued in instalments, tax paid. |
| Year after | Where a PTAB decision or certificate of error lands, refunds issued. |
So the bill you pay this year is for last year's assessment, valued as at 1 January of last year.
The sequence within an assessment year
- 1 January — the valuation date. Everything is assessed as it stood on this day.
- Spring to autumn — townships are reassessed and rolls published, staggered.
- Publication + 30 days — your appeal window. Township by township, so no single date.
- Board of Review hears appeals, into the winter.
- Department of Revenue issues the county equalisation factor.
- County clerk calculates rates from each district's levy.
- Following year — bills issue, typically in two instalments.
Why the valuation date matters so much
Everything is judged as at 1 January. That determines which evidence is relevant:
- A sale in March is after the valuation date — useful, but note the gap.
- A renovation completed in June did not exist on 1 January and should not be in that year's value.
- Damage in September is next year's argument, unless a specific disaster provision applies.
- Occupancy for the homestead exemption is tested as at 1 January.
The lag people find hardest
You appeal in the spring, win in the autumn, and see nothing until a bill the following year. That is not an oversight — the correction is entering a cycle that was already running.
And you must keep paying the bill you have while the appeal proceeds. An appeal does not suspend liability, and interest accrues regardless of how strong your case is.
The two dates to diary
Your appeal deadline, which is on your notice and is not extendable.
Your township's next reassessment, because that is when any correction you win is discarded and the model starts again — and the year to be watching most closely.
Why Illinois pays a year late
The arrears system exists because the amount owed cannot be known until every step has finished. Values must be set, appeals heard, the equalisation factor issued, and every overlapping district's levy adopted, before a rate can be calculated at all.
Only then can a bill be produced — and by that point the year being taxed has ended. Every other state with an assessment-and-levy system faces the same problem; Illinois resolves it by billing afterwards rather than estimating in advance.
What this means when you buy or sell
At closing, property tax is prorated between buyer and seller because the bill for the period being transferred has not yet been issued. The seller credits the buyer for their share of a tax nobody has received a bill for.
The estimate is usually based on the last known bill, so if the assessment has since risen, the credit can fall short and the buyer meets the difference. Ask how the proration was calculated and against which year's bill — it is a real number and it is frequently agreed without anyone examining it.