Appealing a condo assessment in Cook County

How the Assessor values a condo unit

The Cook County Assessor explains it in two steps. First, it uses a statistical model to estimate the total value of the building, built from sales of units in your building and in nearby, similar buildings. Then it applies each unit's percentage of ownership to that building value to set each unit's value.

According to the Assessor, condo values are typically driven by:

• location, such as city, neighborhood and school district;
• characteristics of the building, such as its age and total number of units;
• characteristics of the unit, such as its percentage of ownership relative to other units in the building.

Condos are residential property. The Assessor lists condominiums and cooperatives among Class 2 properties, and the Board of Review refers to residential condominiums as class 2-99.

Your percentage of ownership

The percentage of ownership is not set by the Assessor. It is created when the Condominium Declaration is filed with the County, usually by the building's developer. Each owner should have a copy of the Declaration; if you do not, the Assessor suggests asking your HOA.

You can also look it up yourself. On your property's PIN detail page on the Assessor's site, scroll down to the blue "Access more in-depth property details" button and look for the "Common Interest" percentage. The Assessor also publishes its condo unit characteristics on the Cook County Open Data portal.

Because each unit's value follows its percentage, a unit with a higher percentage of ownership will have a higher value than units with lower percentages in the same building. So when you compare assessments inside your building, the Assessor says to compare your unit with units that have a comparable percentage of ownership.

Filing alone or with your association

The Assessor says condo owners can file on their own or as a group. If the appeal is filed as a group, the results cover all the units. If you file individually, you only get the result for your unit.

Many associations file for every owner in the building, depending on the HOA. You do not have to take part: if your HOA files for the whole building, the Assessor says to contact your HOA to opt out. The Assessor's rules strongly encourage owners to file with their association, but also allow you to opt out and appeal your own unit only. If you do, state clearly in your appeal that you are filing for your individual unit only.

Watch for "double dockets." The Assessor's 2026 guidelines flag the case where appeals are filed for both the entire building and an individual unit. Before you file alone, check with your association's leadership whether a building appeal is already planned or filed.

One more thing from the Assessor: if anyone in your building files, the office reviews the values of the rest of the units too. You may get an updated assessment notice even if neither you nor the building appealed.

Evidence the Assessor looks for

The Assessor's rules say most arguments for lowering the value of a condo building are based on in-building sales analysis. Those sales should be recent, within 2 years of the lien date of the appeal year, and arm's length for fair market value.

• No recent sales in your building? The rules say to choose comparable sales the way they describe for other properties.
• Other arguments still count, such as an appraisal or the recent purchase price of a unit.
• Unit details. Filers are encouraged to list each unit's square footage and number of rooms, bedrooms, bathrooms and half baths. The Assessor says this helps it improve how it assesses condos.
• Association filings must include a copy of the Declaration, or its most recent amendment, showing each unit's percentage of ownership. This applies whether the argument is a wrong percentage for some units or a lower value for the whole building.
• Check your property details. Every filer must review the property details for their PIN on the Assessor's site and, if something is wrong, say so and give the correct information.

You file condo appeals at the Assessor with the Condo/Co-Op online appeal while your township is open. The Assessor says supporting documents must be submitted together with the appeal, before the filing deadline. You do not need a lawyer to file there.

At the Board of Review

After the Assessor, you can appeal again at the Board of Review when your township opens there. Who may file matters. Under Board of Review Rule 1, individual taxpayers may represent property titled in their own names, but entities such as condominium associations must be represented by an attorney.

The Board of Review's FAQ puts it plainly: an association president cannot file for the association. Under the Illinois Condominium Property Act and Board of Review procedure, the association, through its board, must hire an attorney to represent the entire building. Otherwise, unit owners may file their own individually signed appeals, and board members may help gather the data and evidence.

In its 2026 presentation for attorneys, the Board of Review recommends that when one representative handles units in the same building, condo complaints be filed as one association filing rather than many individual unit filings. A representative authorized for the entire building should file one complaint for the whole building.

For your own unit, the Board of Review's general advice applies: a photo of the front of the property is required, and your evidence must be in by the evidence deadline after your township closes. See the Board of Review hearing and evidence and Assessor vs Board of Review.

Before you file

Find your percentage of ownership, ask your association whether a building appeal is planned, and look at recent sales in your building. Your township's page shows its 2026 Assessor and Board of Review windows; see deadlines by township.

A free check of your home against similar homes can show whether your assessment looks out of line. Any appeal can end with no change.