How to find comparable properties for a tax appeal
Start from the county's own records, not from what you know about the neighbourhood.
A uniformity appeal is only as good as its comparables. Finding them is a mechanical exercise with public data, and it takes about half an hour once you know where to look.
What you are looking for
Not properties that sold for a similar price. Properties that are assessed similarly to the way yours should be. You are comparing the county's own numbers against each other.
For each candidate you need five things, all of them public:
- PIN and address
- Class code
- Building floor area
- Improvement assessed value — not the total
- Year built
Where to look, in order
1. Your own block
In Cook County, PINs are geographic. A PIN like 14-21-106-017-0000 breaks into area, sub-area, block, parcel and unit. Properties sharing the first three groups are on your block. Change the fourth group and you are looking at your immediate neighbours.
This is the fastest way to find genuine comparables, and it produces the tightest ones — same street, same era, usually the same builder.
2. The county parcel search
Every Illinois county publishes one. Search by address or PIN, and most allow browsing neighbouring parcels. Cook County's assessor site also lets you search by neighbourhood code and class, which is effectively the model's own grouping.
3. The assessor's own comparables
Many offices will tell you which properties drove your valuation if you ask. That is worth doing before you go looking: if you are going to argue, knowing what you are arguing against beats guessing.
The filter
Work through candidates and keep only those that match on all five:
| Attribute | Acceptable range | Why |
|---|---|---|
| Neighbourhood | Same valuation neighbourhood | The model prices by neighbourhood; crossing one is arguing against a different model |
| Class code | Identical, ideally | Different classes are assessed differently |
| Floor area | Within about 20% | Assessment per square foot is not linear — small properties carry a higher rate |
| Year built | Within a decade or two | Construction era drives quality and depreciation |
| Storeys and style | Match | A ranch and a two-storey of equal area are different buildings |
The arithmetic
For each comparable, and for your own property: improvement assessed value ÷ floor area.
Then take the median of the comparables. That is the level you are arguing you should be assessed at. Multiply it by your floor area, add your unchanged land value, and that total is the number you are requesting.
How many, and how to know when you have enough
Four to six good ones. Twenty loose ones are worse than four tight ones, because a single obviously unsuitable comparable gives the office a reason to doubt the rest.
Here is a useful test. Look at how much your comparables disagree with each other. If they span $12 to $18 per square foot and you are at $16, you have a noisy sample and no finding — you are inside their own scatter. If they cluster between $13 and $15 and you are at $19, that is a real gap.
What to do when nothing matches
Some properties genuinely have no close comparables — an unusual house, a very large lot, a converted building. Widen carefully, one attribute at a time, and say in the filing what you widened and why. An office is far more receptive to "the nearest comparables are slightly larger, so this figure is conservative" than to a set that quietly includes properties nothing like yours.