You sold your house. The buyer is happy. You show up at closing ready to sign a stack of papers and collect your check.
Then you see a line item you were not expecting. It says something about a tax credit. And it is taking thousands of dollars off your side of the sheet.
This is the property tax proration credit. It catches Fox Valley sellers off guard more than any other closing cost, and it is almost never explained before it shows up. Here is what it is, why Illinois works this way, and how the number gets decided.
Illinois pays property taxes a year late
Most bills work the way you expect. You use something, then you pay for it that month.
Illinois property taxes do not work that way. They are paid in arrears, which means one year behind. The tax bill that shows up in your mailbox in 2026 is the bill for 2025. You are always paying for a year that already ended.
This is not a Kane County quirk or a Fox Valley quirk. It is how the entire state has worked for decades.
That single fact creates the problem the proration credit solves.
Why the credit exists
Say you sell your South Elgin house on June 30, 2026.
You lived in that house for the first six months of 2026. Property taxes were building up the whole time you were there. But no bill for 2026 exists yet. That bill will not arrive until sometime in 2027, and by then the house belongs to someone else.
So the buyer is going to open a tax bill that covers six months when you owned the home and they did not.
Somebody has to settle that. The way Illinois handles it is simple: you hand the buyer money at closing to cover your share. That handoff is the proration credit.
You are not being charged extra. You are paying a tax you genuinely owe, just earlier than the county would have asked for it, and to the buyer instead of to the treasurer.
How the number is calculated
The math has three parts.
Part one: the starting bill. Nobody knows what the 2026 tax bill will be, because it does not exist. So the calculation uses your most recent full-year bill instead. Attorneys call this the most recent ascertainable full year tax bill.
Part two: the percentage. Taxes usually go up. To account for that, the credit is set at a percentage of that last bill. Common percentages are 100%, 105%, and 110%. Outside of Cook County, 105% is the more typical starting point, and Kane County sits outside Cook. Cook County closings tend to run higher, closer to 110%, because of how reassessments work there.
Part three: the days. The credit is prorated day for day. You pay for the exact number of days you owned the home during the tax year in question.
Here is how that plays out with round numbers. Say your last full-year bill was $8,000, the contract sets proration at 105%, and you close June 30.
- $8,000 times 105% comes to $8,400
- $8,400 divided by 365 days is about $23 per day
- January 1 through June 30 is 181 days
- 181 days times $23 per day is roughly $4,165
That $4,165 comes off your proceeds. On a house where you were expecting a clean check, four thousand dollars is a real number.
The percentage is negotiable, and most sellers never negotiate it
This is the part worth reading twice.
That percentage is not fixed by law. It is a contract term. Your attorney proposes it, the buyer’s attorney accepts it, counters it, or rejects it, and the two of them settle on a figure.
Go back to the example above. On an $8,000 annual bill with a June 30 closing:
- At 100%, the credit is about $3,967
- At 110%, the credit is about $4,364
That is a spread of roughly $400 on a modest tax bill, decided by a single number in a contract most sellers skim.
Now run the same spread on a St. Charles or Geneva home with a $14,000 annual tax bill and the gap gets a lot wider. In Illinois, property tax bills in the corridor towns are often substantial, which makes the proration percentage one of the highest-dollar terms in the whole contract.
The window to settle this is the attorney review period, the five business days after the contract is signed. That is when the percentage gets set. Once review closes, the number is locked.
Two traps that cost sellers real money
The exemption trap. If you have been claiming an exemption that the buyer will not get, your last tax bill understates what the property will actually cost going forward. A senior freeze is the clearest example. A buyer’s attorney who spots this will push for a higher proration percentage, sometimes well above 110%, because they know the next bill is going to jump.
You cannot make that argument disappear. But you can see it coming instead of getting surprised by it, and you can price the home knowing the credit will be larger than a neighbor’s.
The reassessment trap. If your township is due for reassessment, both sides are guessing. Buyers push the percentage up because they expect a bigger bill. Sellers push it down because they do not want to fund a tax increase they will never benefit from. Neither side knows who is right until the bill lands, sometimes more than a year later.
Some contracts handle this with a re-proration clause, which reopens the calculation once the real bill arrives and settles the difference between the parties. It is fairer in principle. It also means you might get a phone call about money a year after you thought you were finished with the house. Worth knowing before you agree to it.
What this means if you are selling this year
Three things to do before you list.
Pull your last full-year tax bill and know the number. Not your monthly escrow payment, the actual annual bill. That number drives your credit. The Kane County Treasurer’s office can confirm it if you cannot find the paperwork.
Check which exemptions are on the property. Anything the buyer will not inherit is going to come up in negotiation. Better to find out now.
Build the credit into your net proceeds estimate from the start. This is the one that matters most for anyone selling in order to buy. If you are counting on a specific number to fund the down payment on your next house, and you have not accounted for a four or five figure tax credit, your math is off before you begin. Our home value tool is the starting point for that estimate, and the seller resources page walks through the rest of what comes out of your proceeds.
The proration credit is not a fee anyone invented to take money from you. It is a tax you owe for the time you lived in the house. The problem is that sellers meet it at the closing table instead of at the kitchen table, when there is still time to plan around it.
We would rather have that conversation early. Get in touch and we will walk through your actual numbers before you list, tax credit included.
This post explains how tax prorations generally work in Illinois residential closings. It is not legal or tax advice. Your attorney handles the proration terms in your specific contract.
