The county's record of your house is often wrong, and that is your best appeal
Building areas recorded as lot sizes. Years built with two years run together. What we found loading millions of property records — and why a record correction beats a uniformity appeal.
Appeal advice concentrates almost entirely on the argument: find comparable properties, show they are assessed lower, argue uniformity. That is a real argument and it sometimes wins.
There is a better one, and it is much less discussed. Before disputing the county's judgement about your property, check the county's facts about it. In our experience loading millions of records across two states, the facts are wrong often enough to be the first thing anyone checks.
What we found
An entire county's building area was lot area
Lake County, Illinois publishes a building square-footage field. Its median value is 9,821 and its 90th percentile is 105,734. No county has a median house of 9,821 square feet. Those are lot sizes, carried into a column labelled as building area.
The same importer produced correct values for Cook County. DuPage failed identically to Lake.
A year built recorded as 18921989
Connecticut's statewide file contained 18921989 in the year-built field — an 1892 structure with
a 1989 addition, concatenated at some point in the record's life. It is not a plausible year and no
validation caught it before us.
Assessments 1,082% above the neighbours
Our comparison found properties assessed 493%, 608% and 1,082% above their peer group. Nobody is over-assessed elevenfold. Each of those is a record error: a commercial parcel grouped as residential, a whole building's assessment attached to a single condo unit, a transposed digit in a square-footage field.
Why this beats a uniformity argument
The difference is what you have to persuade someone of.
| Uniformity appeal | Record correction | |
|---|---|---|
| What you argue | Your judgement about value is wrong | Your file says 2,400; it is 1,900 |
| Who decides | A hearing, on the balance of evidence | Often a clerk, on the document |
| Typical outcome | About 8.4% off assessed value, one in five times | The full difference, once accepted |
| How long it holds | Until the next reassessment | Permanently — the record is fixed |
That last row is the one that matters most and is almost never mentioned. A uniformity win is reset at your next reassessment; three years in Cook County, four in most of Illinois, five in Connecticut. A corrected square footage stays corrected, and every future assessment is built on the right number.
How to check yours
Get the property record card. Every assessor's office will provide it, usually online and free. It lists everything the county believes about your property.
Check the living area first. It is the single biggest driver of the valuation and the field most often wrong. Compare it against your own measurement, your survey, or the listing from when you bought. Beware of what is included: finished basements and attached garages are counted differently by different offices, and a discrepancy may be a definition rather than an error.
Check bedroom and bathroom counts. Removed a bathroom in a renovation? The record probably still has it.
Check the year built. A house recorded as newer than it is carries a higher value.
Check the classification. A residential property carrying a commercial or multi-family code is assessed on a different basis entirely.
Check the land. Acreage errors are less common than building errors but larger when they happen.
How to fix one
A record correction is generally not an appeal. Most assessor's offices have a separate, quicker process for factual errors, because they would rather fix the file than defend it at a hearing.
Call the office, say what field is wrong, and ask what documentation they need. Usually it is a survey, an appraisal, building permits, or in straightforward cases an inspection. Put it in writing as well as by phone, and keep the correspondence.
Why nobody tells you this
There is no industry in record corrections. A contingency firm earns a share of an appeal reduction; a phone call to the assessor pointing out a wrong square footage generates no fee, so nobody advertises it.
It is still the first thing to do. It costs a phone call, it is more likely to succeed than an appeal, and unlike an appeal it does not expire.