Your PIN, and how to read it
The property index number encodes where your parcel sits. It is also the only identifier that is never ambiguous.
Your Property Index Number is the county's unique identifier for your parcel. It appears on your assessment notice, your tax bill, your deed and your closing documents, and it is the only reliable way to look your property up — addresses are ambiguous, PINs are not.
How to read a Cook County PIN
A Cook County PIN is 14 digits, conventionally written in five groups: 14-21-106-017-1001. Each group means something.
| Segment | Digits | What it identifies |
|---|---|---|
| 14 | 2 | Area — the township or town |
| 21 | 2 | Sub-area — the section within it |
| 106 | 3 | Block |
| 017 | 3 | Parcel within the block |
| 1001 | 4 | Unit — 0000 for a whole parcel, non-zero for a condominium unit |
The last group is the one that trips people. A PIN ending 0000 is a whole parcel. A PIN ending 1001, 1002 and so on identifies individual condominium units within a building, each separately assessed and separately appealable.
Why the structure is useful
Because the first four segments are geographic, PINs that share them are physically adjacent. Parcels on the same block share the first three groups. That makes the PIN a fast way to find genuine neighbours for a uniformity comparison — change the parcel segment and you are looking at the property next door.
It is also how you check that a comparable really is nearby. A property with a different area code is in a different township, assessed by a different part of the model, and is a weaker comparable however similar the house looks.
Outside Cook County
Other Illinois counties use their own formats, commonly 10 digits and often written with dashes in a different pattern. The principle holds: the number is hierarchical and geographic, so parcels with similar leading digits are near each other.
Connecticut, by contrast, has no county assessment layer at all — each of the 169 municipalities keeps its own parcel numbering, so an identifier is only unique when paired with the town.
Where to find yours
- Your tax bill — usually the most prominent number on it.
- Your assessment notice.
- Your deed or closing statement.
- The county's parcel search, by address, if you have none of the above.
The mistakes worth avoiding
Transcribing it wrong. Fourteen digits are easy to mistype, and an appeal filed against the wrong PIN is an appeal about somebody else's property. Copy and paste it where you can.
Appealing the wrong unit. In a condominium, each unit has its own PIN. The building's common areas may have another. Make sure you are appealing yours.
Missing a second parcel. Some properties comprise more than one PIN — a house on one parcel with an adjacent vacant lot on another. Both are assessed, both are billed, and appealing one does nothing for the other.