30 days after your township opensNo single calendar date — the clock starts when YOUR Cook County value notice is mailed (30 days).
Yes — you can appeal your Cook County assessment. The deadline is 30 days after your township opens (rolling schedule), so the clock (30 days) starts the day your notice is mailed. File with the Cook County Board of Review (and the Assessor's office, earlier).
Notice-relative deadline — confirm the exact 2026 date printed on your notice / with the Cook County Board of Review (and the Assessor's office, earlier).
Is my property over-assessed in Cook County?
You're over-assessed when the County's value is higher than what your home would actually sell for — and then you're over-paying every year until you appeal. Cook County appeals run on a ROLLING township-by-township schedule: the Assessor opens each township for ~30 days, and the Board of Review re-opens it later for a second 30-day window. There is no single county-wide date — find YOUR township's open/close dates on the Assessor and Board of Review sites. Cook assesses residential at 10% of market value; over-assessment is checkable against recent sales (Illinois publishes sale price).
Live Cook County parcels whose assessed market value exceeds their recent arm's-length sale price — a real over-assessment signal pulled from the public assessor roll. If your parcel looks like these, you have a strong appeal:
How to appeal your Cook County assessment — and who files it for you
Pull your current assessed value from the Cook County assessor portal.
Find your market value — a recent sale of your home, or comparable nearby sales.
If the County's number is higher, file an appeal with the Cook County Board of Review (and the Assessor's office, earlier) by 30 days after your township opens (rolling schedule).
Present your comparables (and any appraisal) at the hearing.
Most owners who win use a contingency property-tax appeal firm — it costs nothing unless they cut your bill, then takes a share of the first-year saving. About 95% of over-assessed owners never appeal, so the saving is usually left on the table.
What is the Cook County appeal deadline this year?
30 days after your township opens (rolling schedule) (the clock starts when your notice is mailed).
How much does it cost to appeal?
Usually a small filing fee or none; a contingency firm only charges if it wins a reduction.
I got a reassessment notice — what now?
It's the County's new market-value opinion and the start of your appeal window. Check it; appeal by 30 days after your township opens (rolling schedule) if it over-states your home's value.
Will appealing raise my taxes?
In most jurisdictions an appeal can only confirm or lower your assessment for that cycle — but confirm the rules with the Cook County Board of Review (and the Assessor's office, earlier) before filing.
Source & verification
against the official Cook County / IL deadline (cookcountyboardofreview.com · 35 ILCS 200/16-95 (Board of Review); 16-55). These are public deadline facts, not legal or tax advice — confirm your exact date with the Cook County Board of Review (and the Assessor's office, earlier) and consult a professional before filing.