In the south and west suburbs' last reassessment, tax year 2023, the median owner-occupied home's tax bill rose 17.6% over the year before. Nearly two in three, 64.0%, rose more than 10%. The next year, with no reassessment, the median rise was 3.8%.
Those 17 townships are being reassessed again in 2026. This year's notices raised the median home's assessed value 20.6% over its final 2025 value. The tax year 2026 bills that reflect the new values are paid in 2027.
That matters at closing. At most Cook County closings, the seller's tax credit is a percentage of the last full-year bill, because the current year's bill isn't out yet. When the real bill comes in higher, the buyer pays the difference. Here is the arithmetic, and a simple way to flag it early.
Quick Answer
Cook County bills are paid a year behind, so at closing the seller credits the buyer for taxes not yet billed. The Multi-Board contract bases that credit on a percentage of the most recent full-year bill and leaves the percentage blank; 100%, 105% and 110% are common fill-ins. In a reassessment year the real bill can outrun it. On a mid-2026 closing in the south or west suburbs, a 105% credit on a typical home would leave the buyer about $468 short if the bill rises like the median home's did last cycle, and about $1,016 short if it rises as much as the top quarter of homes did. Flag it early, and let the attorneys decide whether to use a higher percentage, recalculate the credit when the real bill arrives, or hold money in escrow.
How the closing credit is set
The contract sets a formula. In the Multi-Board Residential Real Estate Contract 8.0, paragraph 12(a) splits the general real estate taxes at the closing date, using a blank percentage of "the most recent ascertainable full year tax bill." Contracts call this the tax proration. It is final at closing unless the parties agree otherwise in writing. Paragraph 12(b) sets a separate 3% escrow for homes not yet taxed as currently improved, such as new construction.
The percentage is negotiated. Chicago-area contract guides describe 100% as "last year's bill," and 105% as building in an expected increase. One suburban law firm's guide calls 110% customary in Cook County and 105% elsewhere.
"Most recent full-year bill" means the last tax year with both installments issued. The tax year 2025 second installment was due October 1, 2026. For a 2026 closing before that bill came out, the most recent full-year bill was tax year 2024.
Which tax years a 2026 closing covers
| Tax year | Who owned it | Bills | Usual handling at a mid-2026 closing |
|---|---|---|---|
| 2025 | Seller, all year | First installment due April 1, 2026; second due October 1, 2026 | Seller pays the first installment and credits the buyer for the estimated rest. Buyer pays the second. |
| 2026 | Seller through closing day, buyer after | Both installments paid in 2027 | Seller credits the buyer for days through closing. Buyer pays both bills in 2027. |
Tax year 2026 is the reassessed year in the south and west suburbs. And the jump lands late. The first installment, paid in early 2027, is by law 55% of the prior year's total. The new values show up on the second installment.
What the record shows about reassessment years
Median change in owner-occupied tax bills, for the same homes, by area and year:
| Area and tax year | Median change | Share up more than 10% | Share up more than 20% |
|---|---|---|---|
| South and west suburbs, 2023 (reassessed) vs. 2022 | +17.6% | 64.0% | 45.7% |
| South and west suburbs, 2024 (not reassessed) vs. 2023 | +3.8% | 4.6% | 0.6% |
| South and west suburbs, 2023 vs. 2021 (two years) | +19.6% | 67.2% | 49.3% |
| North suburbs, 2022 (reassessed) vs. 2021 | +12.5% | 56.0% | 33.0% |
| Chicago, 2024 (reassessed) vs. 2023 | +10.0% | 50.1% | 33.1% |
The two-year row is the one that matches a 2026 closing. A credit based on the tax year 2024 bill has to cover tax year 2026, two years later, including the reassessment.
The 2026 notices point the same way: in 16 of the 17 townships, 79.0% of homes saw their assessed value rise more than 10%. An assessed value is not a bill. Rates adjust, appeals are pending, and past changes don't predict any one home.
The arithmetic
An illustrative example built on the record: a south suburban home whose tax year 2024 bill was $6,461, the median owner-occupied bill in those townships. The contract uses 105%. Closing is June 30, 2026, so the seller owns 181 days of 2026.
Tax year 2025. The seller paid the first installment, 55% of $6,461, or $3,553.55. The estimated full year is 105% of $6,461, or $6,784.05, so the seller credits the buyer $3,230.50 for the rest. Tax year 2025 wasn't a reassessment year here. If the bill rose a normal 3.8%, the buyer actually comes out $77.53 ahead on this piece.
Tax year 2026. The seller credits 181/365 of $6,784.05: $3,364.15. Here is the seller's true share under three outcomes:
| Tax year 2026 bill vs. 2024 | 2026 bill | Seller's true share (181 days) | Buyer absorbs at 105% | Buyer absorbs at 110% |
|---|---|---|---|---|
| +7.7% (two ordinary 3.8% years) | $6,961 | $3,452 | $88 | none |
| +19.6% (median two-year change, last cycle) | $7,727 | $3,832 | $468 | $308 |
| +36.7% (top quarter starts here, last cycle) | $8,832 | $4,380 | $1,016 | $855 |
At 110%, the 2026 credit is $3,524.34. A closing later in the year puts more days of the reassessed year into the credit, so the gap grows. A higher-priced home with a bigger bill scales the same way.
Flag it early
You don't draft the terms. You can make sure nobody is surprised.
- Know the calendar. The south and west suburbs are reassessed in 2026; Chicago is not. See which townships are in the 2026 reassessment.
- Pull two numbers at the offer stage: the most recent full-year bill and the 2026 assessed value on the notice, against the 2025 value.
- Show the buyer a range, clearly labeled as an estimate, using a table like the one above.
- Raise it with both attorneys before attorney review ends. The contract allows other terms agreed in writing. The options attorneys commonly weigh are a higher percentage, an agreement to recalculate the credit when the tax year 2026 bill is issued (often called a re-proration), or an escrow. Which one, if any, is their call.
- Prepare the buyer for the second installment. A lender may raise the monthly escrow payment after it arrives. Here is an explainer you can forward.
The seller's exemptions are a separate gap. The credit is figured from a bill that shows them, and most won't carry over to the buyer. See the seller's bill versus the buyer's bill.
A note you can send a buyer: "This home is in a township the Assessor reassessed in 2026. The tax credit at closing is based on an older bill, and the 2026 bill, paid in 2027, may be higher. In the last reassessment of the south and west suburbs, most owner-occupied bills rose more than 10% in a year. Let's ask the attorneys how they want to handle it. These figures are estimates, not tax advice."
Where Censum fits
Censum House's Buyer Advantage shows the bill a buyer inherits after the seller's exemptions drop, itemized at the property's rate, under your brokerage's brand. Its Township Tax Calendar shows every Assessor and Board of Review window across the 38 townships, with your clients' townships highlighted. Figures are record-backed estimates, not tax advice. The terms of the closing credit belong to the attorneys.
The numbers behind this article
- Bill changes: Censum's tabulation of PTAXSIM, the Cook County Assessor's Office research database, tax years 2021 through 2024. It compares the same residential parcels in both years: same class, a bill above $0, a homeowner exemption both years, and no change in senior exemption or Senior Freeze status. "South and west suburbs" means the 17 townships reassessed in 2023 and 2026 (271,305 homes in the one-year comparison). The Assessor describes PTAXSIM results as not official.
- 2026 notices: Censum's tabulation of the Assessor's Assessed Values dataset: 2026 mailed values against 2025 final values for the same residential parcels in 16 of the 17 townships (406,285 parcels). Bloom Township had no 2026 mailed values in the copy accessed.
- Example base: the median tax year 2024 bill for owner-occupied homes in those 17 townships. The example is illustrative.
FAQ
Which bill does the closing credit use?
Under the Multi-Board contract, a percentage of the most recent full-year bill: the latest tax year whose second installment has been issued.
What is a re-proration agreement?
An agreement to recalculate the seller's share once the actual bill is issued, and settle the difference. The contract allows the parties to agree to it in writing. The attorneys decide whether to use one.
Is Chicago in the 2026 reassessment?
No. Chicago's next reassessment is 2027. North suburban townships were reassessed in 2025.
When will the buyer see the higher bill?
On the second installment for tax year 2026, paid in 2027. The first installment that year is 55% of the 2025 total, so it won't show the jump.
Can the buyer appeal the 2026 assessment?
If they own the home while their township's Board of Review window is open, yes, and filing is free. A lower 2026 value means a smaller share of the 2026 taxes, which they pay in 2027. Windows are on the Board's own calendar.
Sources
- Multi-Board Residential Real Estate Contract 8.0 (Chicago REALTORS, PDF; member access), paragraph 12
- Borla, North & Associates, Explanation of the Real Estate Contract (customary proration percentages)
- The Chicagoland Lawyer, Multi-Board 8.0 Contract Guide
- Illinois Department of Revenue, seller's share of property taxes at closing
- Cook County Treasurer, due dates
- Cook County Board of Review, Frequently Asked Questions (first installment is 55% of the prior year's total)
- Cook County Assessor, Overview of How Appeals Work (reassessment changes appear on the second installment the following year)
- Censum tabulation of PTAXSIM (Cook County Assessor's Office), tax years 2021-2024.
- Censum tabulation of the Assessor Assessed Values dataset (Cook County open data), 2025 and 2026.
Sources accessed November 11, 2026.
Next step
Put a reassessment-year tax estimate in front of your next south or west suburban buyer before the contract is final. See Censum House
Censum LLC is a data and analytics company, not a law firm and not a tax advisor. Censum is independent and is not affiliated with Cook County or any government agency.