First check if it is worth it.
Then file the right appeal.

Cook County is divided into three reassessment groups: the city of Chicago (reassessed in one year), the north and northwest suburbs, and the south and west suburbs. Each group is reassessed every third year. In off-years, your assessed value is typically carried forward with minor adjustments.

The reassessment year matters. It's when new comparable sales are ingested, when the Assessor's AVM re-runs, and when most of the swings happen. A triennial year is also the best appeal year - the Assessor has just committed to a new number that must be defended, and the data used to build it is freshest.

You'll get a mailed notice - officially the Notice of Proposed Assessed Valuation - from the Cook County Assessor's office, usually between February and September depending on your township. It lists the new assessed value, the property characteristics used to derive it, and your appeal deadline with the Assessor.

Read the property characteristics carefully. Square footage, number of bathrooms, land area, and building condition are entered manually and are often wrong. A mis-recorded feature is one of the cleanest appeals you can file: you provide documentation (MLS, a survey, photos) and the Assessor's office typically corrects it administratively.

There are two separate appeal venues, each with its own window, rules, and success rate. You can file at one, the other, or both - they are independent proceedings.

Censum's pre-filled form targets the Board of Review - the venue where evidence quality matters most. Censum reserves the strongest paid packet for properties that clear the highest-confidence threshold after suppression and eligibility checks. For properties with a straightforward Assessor's Office case (characteristic errors), we flag it in the packet so you can file both.

The Board of Review weighs three lines of evidence, in rough order of weight:

1. Recent arms-length sales of comparable properties. The single strongest argument. If three or four genuinely comparable nearby properties sold recently for materially less than your assessment implies, that is very hard to argue against.
2. Township-level ratio studies and equity. If the effective assessment level in your township is systematically higher than neighboring townships, that's a uniformity argument the BOR has a long history of granting.
3. Assessment history and trend. A year-over-year jump well above the township average requires justification (permit, characteristic change) that the Assessor's office often can't produce. Unexplained spikes are appealable.

Weak appeals rest on anecdote ("my neighbor's house is bigger"), emotion ("my taxes are too high"), or unsupported comparables (different school district, different property class). The BOR has rules against those; they fail at scale.

The Board issues a written decision in roughly 60 to 180 days. If your assessed value is reduced, the Clerk of the County applies the new number to your tax extension. If the tax bill for that year has not yet been issued, the bill is generated at the lower number. If you have already paid, a refund is issued - typically in the 30-60 days following the Clerk's processing.

The reduction persists until the next triennial reassessment in your township - typically two more tax years at the reduced level. That is why the lifetime value of even a small reduction compounds so quickly.