A settlement statement sits on the table. Buyer's agent, seller's agent, two attorneys, a closing coordinator. Everyone signs off on a line called the tax proration credit without much discussion, because it looks like arithmetic, not negotiation. It isn't arithmetic. It's a guess, dressed up in decimal points, about a number that in Arlington Heights this year didn't exist anywhere on paper until a few weeks ago.

Here's why that guess mattered more in 2026 than in almost any recent cycle, and why anyone who closed on a home here between January and late August negotiated that credit against a bill that predates both the current tax year and the local reassessment.

Taxes paid a year behind, on a cycle that just got longer

Illinois property taxes are billed in arrears. The bill you receive this year covers last year's ownership, not this year's. Cook County splits that bill into two installments: the first is a fixed 55 percent of the prior year's total tax, mailed early in the year with no connection to anything current. It's a placeholder. The second installment is the one that actually reflects the year's assessed value, tax rate, and exemptions, and it's normally mailed by July 1 with an August 1 due date.

This year it wasn't. Cook County's second installment bills for tax year 2025 were mailed September 1, 2026, with an October 1, 2026 due date, roughly two months later than the traditional schedule. County officials attributed the delay to a technology overhaul of the property tax system that created what the county's own announcement called a cascade effect through the assessor, treasurer, clerk, and board of review. That means the only bill that could show a real, current number for an Arlington Heights property didn't reach anyone's mailbox until this month.

A reassessment that landed on the same bill

Arlington Heights sits inside Wheeling Township, which also covers Wheeling, Des Plaines, Mount Prospect, Palatine, Prospect Heights, Rolling Meadows, Buffalo Grove, and Northbrook. Wheeling Township goes through the county's standard reassessment every three years, and 2025 was its year. The Cook County Assessor's office mailed reassessment notices to Wheeling Township property owners on August 18, 2025, with an appeal deadline of September 30, 2025. Once those values were certified, they became the numbers built into the tax year 2025 bill, the same bill whose second installment just arrived.

Put those two facts together and the timing problem becomes obvious. The first bill capable of reflecting the Wheeling Township reassessment was always going to be the tax year 2025 second installment. That installment was always going to mail well into the following year, per the standard schedule. This year's two-month system delay pushed it later still, to September 1, 2026. So for roughly the first eight months of 2026, nobody closing on an Arlington Heights home could point to a real number. The only bill on file anywhere was the tax year 2024 second installment, calculated entirely under the prior reassessment cycle from 2022, with no visibility into the new values at all.

Anyone whose closing fell before September of this year negotiated a proration credit against a number that was, by definition, two reassessment cycles removed from reality.

The assessor's own estimate ran behind the market

The Wheeling Township valuation report gives a useful check on how much that lag actually costs. For single-family homes, the assessor's median estimated market value came in at $429,000, while the actual median sale price for 2024 came in at $457,000, a gap of roughly 6 percent. That gap exists even in a normal cycle, because mass appraisal works off recent comparable sales and a market that's still moving pulls ahead of any assessment built on data that's already a year or two old. It's a reasonable margin for a countywide model covering thousands of parcels, but it's also a reminder that the number on a reassessment notice is a compromise, not an oracle. The appeal window exists because the county expects some of these to need correcting, not because appealing is adversarial.

What the percentage on the settlement statement actually buys

Since nobody has a current bill to work from, the industry solves the gap with a percentage. Cook County closings customarily prorate at 105 to 110 percent of the last full year's tax bill, higher than the 100 to 105 percent typical elsewhere in Illinois, because Cook County taxes have historically climbed faster. In a reassessment year, that cushion often moves higher still, into the 120 to 125 percent range, specifically to protect the buyer against a jump that hasn't shown up in writing yet.

The percentage sounds abstract until it's converted into dollars. Current county assessment data puts Arlington Heights' median property tax bill at around $8,400, with an effective rate near 2.52 percent, though the number swings by zip code: roughly $7,414 in 60005 versus $8,795 in 60004, a difference driven by which school district and special service area levies apply to that block. On a bill near that $8,400 median, the daily proration rate runs about $23 at 100 percent, about $25 at the customary 110 percent, and close to $29 at a reassessment-year 125 percent. Multiply that spread across a typical eight-month seller occupancy period and the difference between the low and high end of that range lands north of $1,300. The percentage is a negotiable line in the contract, and it appears on the settlement statement as a single credit, so reviewing it with your attorney during attorney review is the point at which that spread can be discussed.

Exemptions don't follow the house

A separate wrinkle compounds the reassessment timing gap: Cook County's homeowner exemption, senior exemption, and senior freeze belong to the person, not the property. When a home sells, the seller's exemptions come off the tax roll, and the buyer has to apply for whatever they qualify for. If the seller had held a senior freeze or long-standing homeowner exemption, the bill their proration was based on was already reduced by that exemption, and the buyer's real first bill, calculated without it, can come in meaningfully higher than the number used at closing. That gap has nothing to do with reassessment cycles or system delays. It shows up every year, in every township, and it stacks directly on top of whatever timing gap already exists.

What to actually ask for before signing

A few specific requests change the outcome of this negotiation, and none of them require waiting for a bill that doesn't exist yet.

• Ask which exemptions currently apply to the property and whether they're likely to carry forward. A seller's senior freeze or long-time homeowner exemption disappearing is a known, calculable gap, not a surprise.
• If the property is in Wheeling Township, ask specifically whether the reassessment notice was appealed, and if so, what the resolution was. That answer exists now that the Board of Review process for this cycle has closed.
• Push for the proration percentage explicitly rather than accepting a boilerplate default. A reassessment-year home is a reasonable place to negotiate toward the higher end of the customary range.
• Ask whether the contract includes a re-proration clause. In most Cook County transactions, once the closing happens, that proration is final. There's no automatic do-over when the real bill lands higher or lower than what everyone assumed. A re-proration clause is the only mechanism that keeps the door open, and it has to be written in before signing, not requested after the fact.

Frequently Asked Questions

If I already closed on an Arlington Heights home earlier this year, can I revisit the tax credit now that the real bill is out? Only if the contract included a re-proration clause. Without one, the proration agreed to at closing is generally treated as final, regardless of how different the actual bill turns out to be.

Does a reassessment automatically mean a higher tax bill? Not necessarily. A property's actual bill depends on how its assessed value moved relative to the township average, since the total tax levy gets divided across all of Wheeling Township's assessed value. A property that appreciated in line with its neighbors may see a smaller shift than the headline reassessment number suggests.

When does this timing problem go away? Wheeling Township's next scheduled reassessment isn't until 2028, so the specific reassessment-related gap eases for a few years. The structural arrears billing and the exemption-transfer issue, however, apply to every closing regardless of reassessment year.

If you're weighing a purchase or a sale in Arlington Heights right now and want the actual numbers run before you sign anything, Maria Devins can walk through what the current bill, the exemption history, and the proration language on your specific contract mean in real dollars.