If you are buying or selling real estate in Illinois, you will encounter the term “tax proration” at your closing. For many people, it is one of the most confusing line items on the settlement statement. The buyer gets a credit, the seller pays a charge, and somehow the math involves last year’s tax bill multiplied by 105%.
To understand why Illinois handles property taxes this way, you have to go back nearly a century — to the Great Depression. What happened in the 1930s created a system that still governs every residential and commercial real estate closing in the state today.
This guide explains the full story: where the arrears system came from, how it works in practice, how prorations are calculated at closing, and the key differences between Cook County and the rest of the state. Whether you are a first-time homebuyer or a seasoned investor, understanding tax prorations will help you avoid surprises at the closing table.
In This Article
A System Born from the Great Depression
In the early 1930s, the American economy collapsed. Banks failed, unemployment soared, and property values plummeted. Across Illinois, county governments faced a crisis: the property tax system in place at the time required assessors to estimate what a property would be worth in the coming year and then bill the owner based on that estimate. When property values were stable, this worked reasonably well. When the economy cratered, it did not.
Assessed values set before the crash bore no resemblance to what properties were actually worth. Homeowners who had lost their jobs could not pay taxes calculated on inflated pre-Depression valuations. Tax delinquency rates in some Illinois counties exceeded 50%. County treasurers could not fund basic services. The system was broken.
Illinois responded by fundamentally restructuring its property tax collection timeline. Instead of billing taxes based on estimated future values, the state shifted to a system where taxes are assessed and billed based on the prior year’s known property values. This meant that by the time a tax bill was issued, the assessed value had already been finalized, appeals had been resolved, and the tax rate had been set by every local taxing district. The bill reflected reality, not a guess.
The Old System
Assessors estimated future property values and billed taxes in advance. Property owners paid based on projections.
The Crisis
Property values collapsed. Tax bills based on inflated pre-crash estimates became unpayable. Delinquency rates exceeded 50% in many counties.
The Shift to Arrears
Illinois restructured the system so taxes would be billed based on the prior year's known, finalized assessed values instead of forward-looking estimates.
The Modern System
The arrears system remains in effect. The taxes you pay in 2026 are for the 2025 tax year. This one-year lag creates the need for tax prorations at every real estate closing.
That Depression-era decision created what we now call the arrears system — and it has governed Illinois property taxes ever since.
What “Paying in Arrears” Actually Means
In most states, property taxes are paid in the same year they are assessed. Illinois is different. When you pay your property tax bill in 2026, you are paying for the 2025 tax year. The taxes are always one year behind.
Here is the typical timeline for downstate (non-Cook County) Illinois:
Jan 1, 2025
Assessment date. The county assessor determines the fair market value of your property as of this date.
2025
The assessment is finalized, equalization factors are applied, and each local taxing body sets its tax rate (levy).
June 2026
First installment tax bill is due. Typically 50% of the prior year's total tax. This is an estimated payment.
Sept 2026
Second installment is due. This reflects the actual new assessed value and current tax rate. The final bill.
Key Takeaway
If you own property in Illinois on January 1 of a given year, you are the taxpayer of record for that entire tax year — even if you sell the property on January 2. This is precisely why tax prorations at closing are so important: without them, the buyer would inherit the seller’s full-year tax obligation with no compensation.
Why Tax Prorations Matter at Closing
Because of the arrears system, when a property changes hands in Illinois, there is almost always an unpaid tax obligation hanging over the transaction. The seller has been living in the home and benefiting from local government services — schools, roads, police, fire — but the tax bill for that period has not been issued yet.
Without a proration, the buyer would be stuck paying the entire tax bill when it eventually arrives — including the portion that covers the months (or full year) the seller owned the property. That would not be fair.
The Seller’s Obligation
Lived in the property and used local services from Jan 1 through the closing date. Owes taxes for that period — but the bill hasn’t arrived yet.
The Proration
At closing, the seller’s share of the estimated taxes is calculated and given to the buyer as a credit. The seller effectively prepays.
The Buyer’s Benefit
The buyer receives the credit and uses it to pay the full tax bill when it comes due — months or even a year after closing.
How Tax Prorations Are Calculated
The standard Illinois tax proration follows a straightforward formula. In most residential contracts, the proration is calculated at 105% of the most recent tax bill. The extra 5% is a cushion that accounts for the near-certain increase in taxes from one year to the next.
The Formula
Worked Example
Closing date: June 15, 2026 • Prior year (2025) tax bill: $8,000 • Proration rate: 105%
| Step | Calculation | Result |
|---|---|---|
| Estimated Annual Tax | $8,000 × 1.05 | $8,400.00 |
| Daily Rate | $8,400.00 ÷ 365 | $23.01 |
| Days Seller Owned (Jan 1 – Jun 15) | 31 + 28 + 31 + 30 + 31 + 15 | 166 days |
| Seller’s Credit to Buyer | $23.01 × 166 | $3,819.66 |
In this example, the seller receives a $3,819.66 deduction from their proceeds at closing, and the buyer receives that amount as a credit. When the 2026 tax bill arrives in 2027, the buyer pays it in full — using the proration credit to offset the seller’s share.
100% vs. 105% Proration: What’s the Difference?
The proration percentage is one of the most negotiated terms in an Illinois real estate contract. It determines how much of a cushion the buyer gets against a likely tax increase.
100% Proration
- •Based on the prior year’s actual tax bill with no adjustment
- •Favors the seller because the actual bill will almost certainly be higher
- •Buyer absorbs the full difference when the real bill arrives
- •Less common in standard residential contracts
105% Proration
- Adds a 5% cushion to the prior year’s bill
- More closely approximates the actual upcoming tax bill
- Standard in most Illinois residential purchase contracts
- Some contracts specify 110% in rapidly appreciating areas
Impact on the Same $8,000 Tax Bill (June 15 closing, 166 days)
No cushion
100%
Credit: $3,638.36
Standard
105%
Credit: $3,819.66
High-growth areas
110%
Credit: $4,001.10
The proration percentage is a negotiable term. Buyers should push for 105% or higher to minimize the risk of being undercredited. Sellers naturally prefer 100% because it reduces the deduction from their proceeds. Your attorney can advise on what is appropriate for your specific transaction.
Cook County vs. Downstate: Key Differences
Cook County — home to Chicago and roughly 40% of the state’s population — operates on a different billing cycle than the rest of Illinois. This adds a layer of complexity to proration calculations in the Chicago metropolitan area that both buyers and sellers need to understand.
Cook County | Downstate (102 Counties) | |
|---|---|---|
| First Installment | Due March 1 (55% of prior year’s total) | Due June (50% of prior year’s total) |
| Second Installment | Due August 1 (balance based on new assessed value) | Due September (balance based on new assessed value) |
| Reassessment Cycle | Triennial (every 3 years, by township) | Quadrennial (every 4 years) |
| Assessment Level | 10% of fair market value (residential) | 33.33% of fair market value |
| Proration Complexity | Higher — second installment often arrives late and varies significantly | More predictable billing cycle |
Cook County Delays
The second installment bill is frequently delayed and can arrive well past the August 1 due date. This makes proration estimates especially important for closings that occur between installments, because the actual amount is unknown.
Triennial Reassessment Shock
Cook County reassesses every 3 years by township. In a reassessment year, the second installment can jump dramatically — making a 105% proration insufficient. Buyers closing in reassessment years should consider negotiating 110%.
Reproration Clauses: Adjusting After Closing
Because the tax proration at closing is based on an estimate, the actual tax bill may differ from the prorated amount. A reproration clause in the purchase contract allows the parties to settle up once the real numbers are known.
With a reproration clause, after the actual tax bill is issued, the buyer and seller compare the prorated amount to the real obligation. If the proration was too low, the seller owes the buyer the difference. If it was too high, the buyer owes the seller a refund.
With Reproration Clause
- Parties reconcile once the actual bill is issued
- Seller pays the difference if the estimate was too low
- Buyer refunds the excess if the estimate was too high
- Protects both parties from estimation error
- Recommended in reassessment years and new construction
Without Reproration Clause
- The proration at closing is final
- No post-closing adjustment regardless of actual bill
- Buyer accepts the risk of a higher-than-estimated bill
- Buyer benefits if the actual bill is lower
- The 105% cushion partially mitigates buyer risk
Whether to include a reproration clause depends on the transaction. In stable tax environments with a 105% proration, many buyers and sellers agree to make the proration final for simplicity. In reassessment years, new construction, or situations where the tax bill is likely to change significantly, a reproration clause provides valuable protection.
Common Tax Proration Pitfalls
Even with a standard 105% proration, certain situations can result in a significant gap between the estimated and actual tax bill. Watch for these:
New Construction
If the property was recently built, there may be no prior tax bill to base the proration on. The land may have been taxed at a vacant lot rate, meaning the first full tax bill after construction will be dramatically higher. Buyers should insist on a proration based on the estimated improved value, not the vacant lot bill.
Exempt-to-Taxable Transfers
Properties transferring from a tax-exempt entity (such as a church, school, or government body) to a private buyer will have no meaningful tax history. The proration must be estimated from comparable properties, which requires careful analysis by your attorney.
Assessment Appeals in Progress
If the seller has a pending assessment appeal, the tax bill could change retroactively. The purchase contract should address who receives the benefit of a successful appeal and how any tax refund is allocated between buyer and seller.
Homestead Exemptions
The seller may benefit from homestead exemptions (General Homestead, Senior Citizen, Senior Freeze, Disabled Person) that reduce their tax bill. If the buyer does not qualify for the same exemptions, the actual tax obligation will be significantly higher than the prorated amount suggests.
Reassessment Year Surprises
In Cook County, triennial reassessments can cause dramatic jumps in assessed value. A 105% proration based on the pre-reassessment bill may fall far short of the actual bill. Ask your attorney whether 110% or a reproration clause is appropriate.
Special Assessments & SSAs
Special Service Areas (SSAs) and special assessments are sometimes billed separately from the general tax bill. These are easy to miss in proration calculations. Verify whether the property is subject to any SSAs or special assessments that need to be prorated separately.
Frequently Asked Questions
Frequently Asked Questions
Why does Illinois collect property taxes in arrears?
Illinois collects property taxes in arrears as a legacy of the Great Depression. During the 1930s, many counties could not assess and bill taxes quickly enough to keep pace with falling property values and widespread delinquencies. The state adopted an arrears system so that tax bills would be based on known, finalized assessed values rather than estimates. That system remains in place today: the taxes you pay in a given year are actually for the prior tax year.
What is a tax proration at closing?
A tax proration is a financial adjustment made at a real estate closing to fairly divide the property tax obligation between the seller and the buyer. Because Illinois taxes are paid in arrears, the most recent tax bill may not yet be issued at closing. The parties agree to estimate the taxes owed and credit the buyer for the seller's share of the tax year.
What is the difference between a 100% proration and a 105% proration?
A 100% proration credits the buyer based on the prior year's actual tax bill. A 105% proration adds a 5% cushion to account for the likelihood that the current year's taxes will be higher than the prior year. The 105% figure is customary in many Illinois counties because assessed values and tax rates tend to increase year over year. Some contracts may specify 110% in areas with rapidly rising assessments.
How is the daily tax rate calculated for proration purposes?
The daily rate is calculated by taking the estimated annual tax amount (e.g., the prior year's tax bill multiplied by 105%) and dividing it by 365. That daily rate is then multiplied by the number of days the seller owned the property during the current tax year (January 1 through the closing date) to determine the seller's credit to the buyer.
Who gets the tax proration credit at closing?
The buyer receives the credit. Because the seller lived in and benefited from the property during the current tax year but will not be responsible for paying the upcoming tax bill, the seller credits the buyer at closing for the seller's share of those estimated taxes. The buyer will then pay the full tax bill when it comes due.
What happens if the actual tax bill is higher or lower than the proration estimate?
If the actual tax bill is higher than the prorated estimate, the buyer absorbs the shortfall. If it is lower, the buyer benefits from the surplus. In some contracts, the parties negotiate a reproration clause that requires a post-closing adjustment once the actual tax bill is issued. Whether a reproration clause is included depends on the terms of the purchase agreement.
Do all Illinois counties handle tax prorations the same way?
No. While the arrears system is statewide, Cook County operates on a different billing cycle than the other 101 counties. Cook County issues two installment bills per year: the first installment (due March 1) is 55% of the prior year's total tax, and the second installment (due August 1) reflects the actual assessed value and new tax rate. Downstate counties typically issue two installments due in June and September. These differences affect how prorations are calculated at closing.
Can a real estate attorney help with tax proration disputes?
Yes. Tax proration disputes are common, particularly when assessed values change significantly between years, when there is a reassessment, or when new construction is involved. A real estate attorney can review the purchase contract, verify the proration calculations, and negotiate reproration terms to protect your interests.
What is a reproration clause and should my contract include one?
A reproration clause requires the buyer and seller to reconcile the estimated tax proration against the actual tax bill once it is issued. If the estimate was too low, the seller owes the buyer the difference; if too high, the buyer refunds the seller. Whether to include one depends on the transaction. Buyers generally benefit from reproration clauses, especially in reassessment years or when purchasing new construction.
How do homestead exemptions affect tax prorations?
Homestead exemptions reduce the assessed value and therefore the tax bill. If the seller has exemptions (such as the General Homestead Exemption or Senior Citizen Exemption) that the buyer will not qualify for, the buyer's actual tax bill will be higher than the prorated amount suggests. The proration should ideally account for this difference, or the buyer should budget for the increase.