Can I appeal a prior year’s assessment?

In most cases, you cannot file a regular property tax appeal for a prior year once the appeal deadline has passed.

Cook County property tax appeals must be filed during the official appeal window for your township. If the deadline closes, the normal appeal process is no longer available for that tax year.

However, there may still be an option to correct certain errors through a Certificate of Error.

When a Prior Year Assessment Can Be Corrected

A Certificate of Error (COE) may allow a correction if the property assessment contains a verifiable factual mistake.

Examples of qualifying errors may include:

• Incorrect building size or square footage

• A structure listed that does not exist

• A building that was demolished but still included in the assessment

• Incorrect property classification

• Incorrect number of units or property characteristics

If the county confirms the error, the assessment may be corrected.

How Far Back Corrections May Apply

In Cook County, a Certificate of Error may apply to:

• The current tax year

• Up to three prior tax years, depending on the circumstances

If approved, the correction may result in a refund for property taxes that were overpaid.

What Cannot Be Corrected With a Certificate of Error

A Certificate of Error generally cannot be used simply because:

• You believe the property value was too high

• Market conditions changed

• You missed the appeal deadline

Certificates of Error are typically limited to clear factual errors in the property record.

How to Apply for a Certificate of Error

Property owners can apply through either:

• The Cook County Assessor’s Office, or

• The Cook County Board of Review

The application typically requires:

• Identification of the property (PIN)

• Documentation proving the error

• Supporting records such as surveys, permits, or photographs

Key Takeaways

• Prior year assessments usually cannot be appealed after deadlines pass.

• A Certificate of Error may correct factual mistakes in property records.

• Corrections may apply to the current year and up to three prior years.

• Approved corrections may result in a refund for overpaid taxes.

• Both homeowners and commercial property owners in Cook County can apply.

If you believe a past assessment included a factual error, reviewing your property records and applying for a Certificate of Error may help correct the issue and recover overpaid property taxes.