Generally no—you can't file a regular appeal for a prior year after the deadline passes. But a Certificate of Error may correct a verifiable factual mistake (wrong square footage, non-existent/demolished structure, misclassification, wrong unit count), applying to the current year and up to three prior years, possibly with a refund. A COE can't be used merely because you think the value was too high or you missed the deadline.
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Can I appeal a prior year’s assessment?
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In 2025 our clients were 1.7 times as likely to win a reduction at the Assessor, and 1.3 times as likely at the Board of Review, as clients of attorneys who filed 100+ residential appeals (2.2 times at the Assessor in 2026 so far).
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Source: Cook County Assessor and Board of Review public appeal records. Past results do not guarantee a similar outcome.
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