Your assessed value is not your home's value, and the gap is the point
Cook County assesses homes at 10% of market. The rest of Illinois uses 33⅓%. Connecticut uses 70%. Confusing these is the most common reason an appeal goes nowhere.
Here is a mistake that costs people real money, made in both directions.
An owner in suburban Cook County opens their notice, sees an assessed value of $32,000, knows perfectly well their house would sell for around $320,000, and concludes the county has dramatically under-assessed them. They file nothing. In fact they are assessed at exactly the level the ordinance specifies, and whether they are over-assessed relative to their neighbours — the question that actually matters — is something they never looked at.
Or the reverse. An owner in DuPage sees $110,000, believes that is the county's opinion of what the house would sell for, and files an appeal claiming the house is worth $330,000 so the county is wildly wrong. They have just told the assessor their property is worth exactly what the assessor already thinks. The appeal goes nowhere and they conclude the system is rigged.
Both mistakes come from the same place: assessed value is a legally defined fraction of market value, and the fraction is different depending on where you are.
The three numbers
Fair cash value is what your property would sell for in an arm's-length transaction between a willing buyer and a willing seller. This is the standard Illinois law sets. Everyday speech calls it market value.
Assessed value is fair cash value multiplied by a ratio set in law. It is the number on your notice and the number an appeal argues about.
Equalised assessed value is assessed value multiplied by a state equalisation factor, and it is what your tax rate is actually applied to. Most people never see this number and it is doing a great deal of work.
The ratios, and how much they differ
| Where | Ratio | Equaliser | Authority |
|---|---|---|---|
| Cook County, Illinois | 10% | 3.0355 | County classification ordinance |
| Illinois, everywhere else | 33⅓% | ≈1.0 | 35 ILCS 200/9-145 |
| Connecticut | 70% | n/a | CGS § 12-62a |
| South Carolina (owner-occupied) | 4% | n/a | S.C. Code § 12-43-220(c) |
| South Carolina (other) | 6% | n/a | S.C. Code § 12-43-220 |
A $300,000 house carries an assessed value of $30,000 in Cook County, $100,000 in DuPage, and $210,000 in Connecticut. Same house, same value, three numbers that differ by a factor of seven.
None of that means the tax differs by a factor of seven. The rate applied to each is set to fit the base it is applied to. This is the crucial and counter-intuitive point: the ratio tells you almost nothing about whether your taxes are high. It only tells you how to read the number on your notice.
Why Cook County is different, and why it matters to an appeal
Cook operates a classification ordinance under home rule. Rather than assessing everything at the same fraction, it assesses residential property at 10% and commercial and industrial property at 25%. The policy intent is to shift burden toward business.
Because 10% is far below the statutory third used everywhere else, the Illinois Department of Revenue applies a large equalisation factor to bring Cook back into line for the purposes of overlapping districts and state aid. For 2024 that factor was 3.0355.
This has a consequence people rarely notice. In Cook County, a dollar of assessed reduction is worth about three times what it is worth in a county assessing at a third, because it is multiplied by the equaliser before the rate touches it.
We verified this rather than taking it from the statute: every DuPage parcel in our data that carries both an assessed and a market figure returns exactly 0.3333. The statute and the roll agree.
The arithmetic to do before you appeal
Take your assessed value, divide by your jurisdiction's ratio, and ask honestly whether you would list the property at that price tomorrow.
In Cook: assessed ÷ 0.10. An assessment of $32,000 implies the county thinks your house is worth $320,000.
Elsewhere in Illinois: assessed ÷ 0.3333. An assessment of $110,000 implies $330,000.
If the implied value is above what you would realistically list at, you have a market-value argument and should gather evidence of what similar properties actually sold for.
If the implied value is at or below what you would list at — which is the common case, because assessors tend to be conservative — a market-value appeal is not your argument. This is the moment most people stop, and it is the moment they should not.
The argument that survives a fair valuation
There is a second, entirely separate ground, and it does not care what your house is worth.
A uniformity or unequal-assessment claim says: whatever my property is worth, comparable properties near me are assessed at a lower level per square foot than I am, and I should be brought into line with them.
It is a claim about consistency rather than value, and it is available even when the county's opinion of your market value is entirely reasonable. It is also, in our experience of the Cook County decision record, the ground most owners never think to raise — they check whether the value is right, find that it roughly is, and never ask whether it is even-handed.
The arithmetic is: your building's assessed value divided by your building's floor area, compared against the same figure for comparable properties in your neighbourhood. Note that it is the improvement assessment over the improvement area. Dividing total assessed value — land plus building — by the building's floor area is a formula for nothing at all, and it penalises any property on a larger-than-average lot.
Reading your notice properly
- Find the ratio for your jurisdiction. It is in statute, and your assessor's website will state it.
- Divide your assessed value by it to get the county's implied opinion of market value. This is the only number you can sensibly compare against what you know about your house.
- Check the land and building split. Uniformity arguments run on the building figure alone.
- Check the equaliser if you are in Illinois. It determines what a reduction is actually worth and it is published annually by the Department of Revenue.
- Check the square footage on file. A surprising share of successful appeals turn on nothing more elaborate than a floor area that was never correct.
None of this requires an expert. It requires knowing which fraction applies to you — and that single piece of information is the difference between an appeal that engages with the county's actual reasoning and one that argues past it.