Property Tax Prorations at Closing in Illinois: How the Credit Is Calculated and Why It Matters

Illinois property taxes are paid a year in arrears, meaning that the tax bill issued in a given calendar year reflects taxes owed for the prior year. This single fact makes property tax prorations at closing one of the most frequently disputed line items in Illinois real estate transactions. The seller owes taxes for the portion of the year they owned the property, but the bill for that year will not arrive until after closing. Prorations are the mechanism by which the buyer and seller settle this in advance.

Done correctly, prorations produce a fair division of the tax burden that protects both sides. Done incorrectly, they create surprises when the actual bill arrives, and those surprises often turn into disputes between parties who no longer have any contractual relationship.

How Illinois taxes accrue and are billed

In Cook County, property taxes for a given tax year are billed in two installments. The first installment is due in March of the following year and is fifty-five percent of the prior year's total tax. The second installment is due in the summer or fall of the following year and reflects the actual current-year tax, adjusted for the first installment already paid. In practice, Cook County taxpayers pay 2025 taxes during 2026, and do not know the exact amount of those taxes until the second installment bill is issued.

In the collar counties, including DuPage, Will, Lake, and Kane, the billing structure is slightly different but the core principle is the same. Taxes are paid in arrears, and the total for a tax year is not known until well into the following year.

For a real estate closing, this creates a timing mismatch. A property that closes in June 2026 has accrued tax liability for the first half of 2026, but that liability will not be billed until 2027. Unless the seller credits the buyer at closing for the portion of 2026 taxes they owe, the buyer will later pay a full-year 2026 tax bill that includes taxes for months the seller actually owned the property.

The standard proration methods in Illinois

Illinois residential contracts typically use one of two proration methods.

The first is the one hundred percent method, which credits the buyer based on the most recent full-year tax bill, prorated to the date of closing. If the 2024 tax bill was twelve thousand dollars and the property closes on June 30, 2026, the seller credits the buyer for roughly half of twelve thousand dollars for 2025 taxes that have not yet been billed, plus a pro rata share of the estimated 2026 taxes from January through closing.

The second is the one hundred five percent method, often written as "one hundred five percent of the most recent tax bill." This assumes that next year's tax bill will be five percent higher than the current one and bakes that assumption into the credit. In markets where assessments have been rising, as they have in much of Cook County recently, the one hundred five percent method is closer to accurate than the straight one hundred percent method. In flat or declining markets, it over-credits the buyer.

The specific percentage and the proration date are negotiable. A buyer expecting a reassessment or a tax increase may push for a higher multiplier. A seller expecting stable or declining taxes may push for one hundred percent. The contract language governs.

Reassessment years create the largest disputes

Cook County reassesses one-third of the county every year on a rolling basis. A property that is reassessed in the year of closing may see its assessed value change substantially, which means the tax bill based on that new assessment will differ substantially from the prior year's bill on which the proration was based. Both sides can be surprised.

A buyer who closes on a property in a reassessment year with a one hundred percent proration from the prior year's bill may find that the actual new bill is twenty or thirty percent higher, and the seller's credit covered only the prior year's rate. A seller who credits based on one hundred ten percent of the prior bill may later learn that the actual bill came in at the prior year's level or lower, meaning they over-credited.

The contract can address this with a post-closing reproration clause, which requires the parties to recalculate the proration once the actual bill is issued and settle any difference. Reproration clauses are not standard in every form contract, and a buyer or seller who wants one must add it specifically. Without it, whatever proration was calculated at closing is final, regardless of how the actual bill compares.

Exemptions and the buyer's first-year tax bill

A second common surprise involves exemptions. The Cook County homeowner exemption, senior exemption, and other exemptions follow the individual rather than the property. When a property is sold, the seller's exemptions come off the new bill, and the buyer must apply for any exemptions to which they are entitled.

The result is that the buyer's first-year tax bill may be meaningfully higher than the prior year's bill, not because the property was reassessed, but because exemptions that were reducing the prior bill no longer apply. If the proration at closing was based on the prior, exemption-reduced bill, the buyer will be short when the actual, non-exempt bill arrives.

A careful buyer's attorney reviews the prior tax bill for exemptions, identifies which ones the buyer will and will not qualify for, and either adjusts the proration accordingly or adds language requiring adjustment when the actual bill issues. Sellers should also confirm that exemption status is accurately represented in the disclosures, because misrepresentation can support a post-closing claim.

Commercial and investment property prorations

For commercial and investment properties, prorations are often more complex because the underlying tax bills are larger and the possibility of a substantial reassessment is higher. Commercial contracts typically negotiate both the percentage multiplier and a reproration clause with a defined settlement period after the actual bill is issued.

Investors acquiring Cook County property should also plan for a possible property tax appeal in the first year of ownership. The purchase itself is often evidence the assessor uses to justify a higher assessment in the next reassessment cycle. An appeal filed promptly can counter that move, but it needs to be coordinated with the closing rather than initiated months later.

When to call a Chicago real estate attorney

Property tax prorations are usually a small section of the closing statement, but they are a section where a buyer or seller can easily lose thousands of dollars through a method that seemed standard at the time. Younis Law Group reviews proration calculations, advises on contract language, and handles post-closing reproration disputes for Chicagoland clients. If you are in contract or approaching closing and the proration section looks routine, take fifteen minutes to confirm it actually reflects your position.