A Certificate of Error is how Cook County fixes a property tax bill after it has been issued. An appeal changes a future bill; a Certificate of Error changes a past one, and the Cook County Treasurer refunds the difference on anything you already paid. It is the right tool when the bill is wrong for a reason that has nothing to do with market value: an exemption you qualified for was not applied, or the county's description of the property was factually wrong.
What it corrects
- Missing exemptions. Homeowner ($10,000 of equalized value), Senior ($8,000), Senior Freeze, Persons with Disabilities, Veterans with Disabilities, Returning Veterans, Home Improvement. This is by far the most common Certificate of Error. The Assessor accepts exemption Certificates of Error for tax years 2021 through 2025.
- Factual errors. Wrong square footage, wrong number of units, a demolished building still assessed, a property classified incorrectly. These are issued by the Assessor and, for assessed-value corrections, reviewed by the Board of Review.
- Court and Board decisions that reduce an assessment after the bill went out are also processed as Certificates of Error.
What it does not cover: "my assessment is too high compared with the neighbors." That is a uniformity appeal, filed in your township's window at the Assessor or Board of Review. See how to appeal.
How it works
- Apply with the Assessor. Exemption Certificates of Error are filed online with the Cook County Assessor with the same proof as the original exemption: ID showing the address, occupancy on January 1 of each year claimed, and for the Senior Freeze, household income for each year.
- The Assessor issues the certificate and sends the corrected figures to the Treasurer.
- The Treasurer refunds. If the Treasurer's records show who paid, the refund is issued without an application. Otherwise you file the Treasurer's Certificate of Error refund application with proof of payment (a cancelled check, card statement, or your lender's Form 1098 if the bill was escrowed). The Treasurer accepts refund claims for Assessor-issued certificates for twenty years from the date the right to the refund arose.
- Check status on the Treasurer's site under Refunds, using your PIN.
How much a missing exemption is worth
The Homeowner Exemption removes $10,000 of equalized value. At Chicago's 6.85% rate that is $685 a year; at Worth Township's 11.28% it is $1,128; at Thornton's 19.02% it is $1,902. Tax years 2021 through 2025 of a missing Homeowner Exemption in Worth come to about $5,640. The Senior Exemption ($8,000) adds 80% of those figures. A Senior Freeze missed in a reassessment year is worth more than either, because it holds the whole equalized value at the old base. See 2025 rates by township for your own multiplier.
When to check
The second-installment bill, due October 1 for tax year 2025, prints every exemption applied. Read the exemptions box. Common reasons an exemption is missing: you bought the home and the previous owner's exemptions dropped off; you turned 65 and never filed for the Senior Exemption; you qualified for the Senior Freeze but did not reapply, which it requires every year; a refinance or a trust transfer reset the record. Anyone who has owned and lived in a Cook County home for several years without checking the exemptions box should check it once.
Our free property check shows the exemption history on your PIN year by year, so a gap is visible in seconds. For the assessment itself, the appeal windows are on the deadlines page. Procedures are from the Cook County Treasurer's refund pages and the Assessor's exemption pages as of September 6, 2026.
