Why two identical houses end up assessed differently
Not corruption. Model drift, stale records, and errors nobody has a reason to find.
Two houses built by the same developer, in the same year, to the same plan, on the same street, carry different assessed values. This is extremely common, and it is the single most fertile source of winnable appeals.
Why it happens
Mass appraisal models are fed by property records, and property records drift apart over decades.
The recorded characteristics differ. One house had a permit for a finished basement in 1998 and the other did not — but the second one finished the basement without a permit, or the first never actually finished it. One has a deck on file. One was re-measured at some point and the other carries a figure from the original construction.
One was appealed and the other was not. This is the big one. In Cook County roughly 440,000 appeals were decided for tax year 2024, and about a fifth of them produced a reduction. Every one of those reductions created a gap between a property that appealed and an identical one that did not. Those gaps persist until the next reassessment rebuilds everything.
Permits landed differently. A kitchen renovation that was permitted raises the recorded quality; an identical one done without a permit does not.
They fell on different sides of a boundary. Valuation neighbourhoods are polygons. Two houses facing each other across a street can sit in different ones and be priced by different parts of the model.
Sales happened at different times. A property that sold recently may have been picked up by the model directly; one that has not sold since 1994 is valued by inference.
Why this is good news for you
Because the disparity itself is the argument. You do not have to prove the market value of your house. You have to show that a genuinely comparable property is assessed at a lower level per square foot than yours, and ask to be brought into line.
Where the houses are near-identical — same builder, same plan, same street — the comparison is about as clean as this process ever gets, and it is very hard for an office to explain away.
How to check in fifteen minutes
- Find your parcel page on the county assessor's site and note your improvement assessed value and your recorded floor area.
- Divide one by the other. That is your assessment per square foot.
- Look up four or five neighbours with the same house type — in Cook, PINs sharing your first three segments are on your block.
- Do the same division for each.
- Compare. If yours is meaningfully above the group, you have a uniformity argument.
What counts as meaningful
Not every gap is worth filing on. Assessors round, records are a year stale, and a few percent is inside the noise of the process itself.
The tighter the comparison group, the smaller the gap that means something. Where you have four near-identical houses on one street and yours is 12% above the others, that is a real finding. Where your "comparables" are scattered across a mixed neighbourhood and vary widely among themselves, a 12% gap is ordinary scatter and an office will say so.
As a rule of thumb: if the properties you are comparing against disagree with each other by more than the amount you are above them, you do not yet have an argument — you have a noisy sample. Tighten the group before you file.
The record check that comes first
Before concluding you are over-assessed, check whether the difference is explained. Pull the recorded characteristics for each comparable, not just the assessment. If the house next door is assessed lower because the county thinks it is 300 square feet smaller, and it genuinely is, there is no disparity — and if it is not smaller, you have found an error in their record, which is a different conversation.