Your school district decides most of your bill
Schools commonly take the majority of an Illinois property tax bill, which is why identical houses differ.
If you want to understand why two similar houses in the same county have very different tax bills, the answer is almost always the school district.
The share
Schools typically account for 60 to 70% of an Illinois property tax bill. Everything else — municipality, county, park, library, fire, township, community college — divides the remainder.
So a difference in school district rates dominates every other difference between two bills.
Why rates differ so much
A district sets a levy, the total dollars it needs. The rate is that levy divided by the total equalised assessed value in the district.
Two districts with identical budgets and identical numbers of pupils will have very different rates if one has a large commercial tax base and the other does not.
This is the mechanism, and it is worth sitting with. A district containing a shopping centre, an office park or an industrial estate spreads its levy across a much larger base. A residential-only district spreads the same levy across houses alone.
The result is that the areas with the least commercial property often have the highest residential rates — which is the opposite of what most people assume.
Why Cook rates vary three to one
Cook County composite rates run from roughly 6.5% in parts of Chicago to over 20% in some south suburban areas. Almost all of that spread is the tax base, not the spending.
Communities with weak commercial bases and significant needs end up with the highest rates, which then makes them less attractive to the commercial development that would relieve them.
What this means for your appeal
Two things, and the second is often overlooked.
You cannot appeal the rate. The Board of Review decides whether your assessed value is correct and even-handed. It has no authority over any district's levy.
But the rate determines what an appeal is worth. The same $2,500 assessed reduction is worth about $190 a year at a 6.5% rate in Cook and nearly $600 at 20%. If you are in a high-rate area, an appeal is worth proportionally more of your evening.
Where to look
Your tax bill breaks the composite rate down by district. Read it once — most people never do — and you will see immediately which body drives your bill and by how much.
If you want to change the levy rather than your share of it, that is a school board question, decided at public meetings, and it is a different activity from appealing an assessment.