How the check works

Where the data comes from

Everything comes from Cook County’s open data catalog (datacatalog.cookcountyil.gov), read when you run the check: parcel addresses, assessed values, building characteristics, and the Assessor’s and Board of Review’s appeal records. We never ask the county for owner or appellant names, and never show them.

The comparison

• Similar homes: same property class and assessment neighborhood, building size within 20%, built within 15 years; at least 5 of them.
• For each similar home: its assessed value divided by its building square feet. Their median, times your building’s square feet, is what similar homes would suggest for yours.
• Your home is higher than similar homes when it is at least 10% above that, lower when at least 10% below, and in line in between.
• The comparison uses one stage of the year for every home: after the Board of Review if the Board has ruled, else the Assessor’s final value, else the value on the notice. A neighbor’s notice is never set against your final value.
• Market value here is the assessed value ÷ 10%: Cook County assesses homes at 10% of market value.
• The 6 similar homes closest to yours in size (then age) are listed as examples.

The tax estimate

Where the Assessor’s tax figures for a parcel are on file (PTAXSIM, refreshed when the Assessor publishes a new year), the check turns the assessment into dollars: the assessed value × the state multiplier, less the parcel’s exemptions, × its tax code’s rate, all from the latest published tax year, with the rate adjusted for how much the tax code’s home assessments have changed since (levies change little, so when assessments rise, rates fall about as much). For a home assessed above similar homes it shows the same sum at the similar homes’ level, and the difference. It is an estimate: the rate and the multiplier are set again every year. There is no dollar figure for a home with the senior assessment freeze. The appeal packet is offered when the estimated difference is at least $150 a year (or, without tax figures, whenever the home is above similar homes).

When the check cannot answer

It says why instead of showing a blank or a zero: no parcel at that address, several parcels and no unit, the county’s service did not answer, too few similar homes, no building size on record, or a condominium (not checked yet).

Past outcomes

For your township we count the appeals filed by owners themselves (no agent or attorney) in recent years, and how many got a reduction, at the Assessor and at the Board of Review. Only decided appeals are counted (those still open are neither won nor lost), and a share is shown only on 20 decided appeals or more. These are owners who chose to appeal, likely with better cases than average: past outcomes, not a forecast for your home.