How the Morganwell property tax check works

Written by Catherine JP, founder. Data last refreshed October 20, 2026.

What the check answers

Whether the county’s 2026 value for a home is higher than the public records support, judged the way the Cook County Assessor and Board of Review ask homeowners to argue it: against similar homes, against recent sales, or against the home’s own purchase price. It is an estimate from public records. It is not an appraisal, and the county decides every appeal.

Data sources

• Cook County Assessor open data: Assessed values (mailed, certified and Board of Review), property characteristics, condominium unit shares, parcel addresses and recorded sales.
• Cook County Clerk tax rates: The composite tax rate for each tax code, used to turn a lower assessment into a yearly dollar estimate.
• Illinois Department of Revenue equalizer: The state equalization factor for Cook County (3.0300 for the 2025 bills).
• Cook County Assessor appeal calendar: When each township opens and closes for Assessor appeals.
• Cook County Board of Review dates: Board of Review windows and evidence deadlines by township.

The four kinds of evidence

1. Similar homes assessed lower (lack of uniformity). Homes in the same assessor neighborhood and class, within 20% of the size and 15 years of the age (25 years when a neighborhood is small), compared by building value per square foot. At least 8 such homes are needed, and the home must sit at least 3% above the comparison point.
2. Similar homes sold for less (overvaluation). Recent arm’s-length sales of comparable homes nearby. Foreclosures, bank sales and family transfers are left out. Sales of homes since rebuilt or enlarged are left out.
3. The home’s own recent purchase. A verified arm’s-length purchase below the county’s value. Owners must disclose a recent sale either way: the Assessor asks about the last two years, the Board of Review the last three.
4. Condominium building sales. At least 3 recent sales of other units in the same building, each scaled by the unit’s share of the building, within 18 months of the January 1 assessment date. Sales far from the building’s middle are dropped.

Guardrails

• A comparison-based ask is capped at 25% off the county’s value. Asks above 40% are treated as data errors and not shown.
• No ask goes below the home’s own recent sale price, because the county sees that sale.
• New construction (built in the last two years) is not compared with older homes.
• When there are not enough comparable homes or sales, the check says so instead of guessing.

How the yearly saving is estimated

Assessed value reduction × the state equalizer (3.0300) × the home’s own composite tax rate from the County Clerk. Exemptions, tax-rate changes and the county’s decision all change the real number, so it is shown as an estimate.

Limits

• The records do not show interior renovations, so a rehabbed home can look over-assessed next to unrenovated neighbors.
• Square footage and characteristics are the county’s; if they are wrong, the comparison is too (that error is itself grounds to appeal).
• Morganwell covers residential homes, 2 to 6 unit buildings, townhouses and condominium units in the Cook County townships listed in the data table.

Updates

Each township is reloaded from the county’s open data when the Assessor mails its notices, again when values are certified, and whenever the rules change. Every change is checked against the largest results by hand before it goes live.

Questions or a correction: info@morganwell.com. Morganwell is not a law firm and does not give legal advice.