How Cook County property assessments work

Reassessment happens on a three-year cycle

Cook County is split into three areas — the city of Chicago, the north suburbs, and the south and west suburbs — and each is fully reassessed once every three years, on a rotating schedule. Your township gets reassessed, then sits for two years before its next full reassessment. That’s why some years bring a notice in the mail with a new value on it, and other years don’t.

A reassessment year is also when the biggest swings happen. The Assessor’s office re-estimates the market value of every property in the township at once, using recent sales and property characteristics. If home prices in your area have climbed since the last reassessment, your new assessed value usually climbs with them — sometimes by a lot in one step.

Assessed value is 10% of market value, for homes

Illinois lets Cook County assess different property types at different percentages of market value — a system called classification. Homes (Class 2 residential property, one to six units) are assessed at 10% of their estimated fair market value. So if the Assessor believes your home is worth $400,000, your assessed value should land around $40,000.

This matters because your assessed value is really just a stand-in for the county’s opinion of your home’s market value. You can work backward: divide your assessed value by 10% (that is, multiply by 10) and you get the market value the county is implicitly assigning to your home. That implied market value is the number worth checking — it’s usually easier to reason about than the assessed value alone, and it’s the number an appeal actually argues about.

The equalizer and tax rate are applied after

Two more numbers turn your assessed value into an actual bill, and neither of them is something you can appeal.

First, the state applies an equalization factor (sometimes called the multiplier) to Cook County’s assessed values, to bring the county in line with the rest of Illinois. This factor has generally been above 3.0 in recent years, which is why you’ll sometimes hear it described as roughly tripling assessed value into what’s called the equalized assessed value, or EAV.

Second, your local tax rate — set by your township, school district, municipality, and other overlapping taxing bodies, based on how much revenue they need — is applied to your EAV (after exemptions) to produce your actual bill.

Neither the equalizer nor the tax rate has anything to do with your individual property. They’re applied uniformly across the county or your local district. That’s exactly why an appeal can only target one thing: the Assessor’s estimate of your property’s assessed value — not the tax rate, and not your bill directly.

Why your bill can rise even when nothing changed at your house

Because the equalizer and tax rate move independently of your home, your bill can go up even if the Assessor’s opinion of your home’s value stayed exactly the same — if the equalizer rose, or your local taxing bodies raised their levies. Conversely, a successful appeal that lowers your assessed value won’t necessarily produce a proportional drop in your bill, since the equalizer and rate still apply to whatever assessed value results.

What an appeal can do is correct the one number that is about your property specifically: whether the Assessor’s estimate of your home’s value is actually in line with comparable homes and recent sales. That’s the entire scope of a property tax appeal in Cook County — and it’s covered in detail in our guide to how to appeal your Cook County property taxes yourself.