Your assessment is not a fact. It is a claim someone made about your house, usually
without visiting it, from a model applied to a whole township at once. This page shows exactly how
that number is built and exactly how you check it.
Part 1 — How the number on your notice is made
1The assessor estimates market value
Not by inspecting your house. Illinois assessors use mass appraisal: a statistical model over recent
sales in your neighbourhood, adjusted by characteristics they hold on file — square footage, age,
class, lot size. If any of those characteristics are wrong on their card, the model's output is
wrong, and it stays wrong until somebody says so.
2They apply the statutory level of assessment
Outside Cook County, Illinois assesses at 33⅓% of market value. So a home the
assessor believes is worth $300,000 carries an assessed value near $100,000. Cook County uses its
own classification ordinance, with residential property at 10%.
3The state equalisation factor is applied
The Illinois Department of Revenue issues a multiplier per county so that assessment levels are
comparable statewide. Assessed value × the equaliser = your Equalised Assessed Value
(EAV). This is the number the tax rate is actually applied to.
4Exemptions come off
Homeowner, senior, senior freeze, veterans with disabilities, home improvement. Each reduces EAV,
not the tax rate. A missing exemption is often worth more than an appeal and costs nothing to claim.
5Every taxing body's rate is applied
Schools, municipality, county, park district, library, fire protection — each sets a levy, and the
combined rate is applied to your net EAV. In much of Illinois this lands between 7% and 12% of EAV.
This is why the assessment matters so much here: Illinois has among the highest
effective property tax rates in the country, so a modest reduction in assessed value is worth more
than it would be almost anywhere else.
Assessor's estimated market value$300,000
× statutory level of assessment (33⅓%)$100,000
× county equalisation factor (say 1.0)$100,000
− homeowner exemption−$6,000
= net equalised assessed value$94,000
× combined tax rate (say 8.5%)$7,990 a year
Part 2 — The two ways an assessment can be wrong
It can be wrong about the market
The assessor says your house is worth $300,000 and comparable homes are selling for $250,000.
This is the argument most people think of, and it is the harder one: it needs comparable sales,
adjustments for differences between those houses and yours, and it invites an argument about
whose opinion of value is better.
It can be wrong about your neighbours — and this is the stronger ground
The Illinois Constitution requires that property be taxed uniformly. If comparable properties in
your own neighbourhood are assessed at a lower rate than yours, that is a ground for relief
in its own right — no opinion of value required, no appraisal, no argument about
the market. You are not saying the assessor is wrong about what houses are worth. You are saying
they treated you differently from the house next door.
This is the argument almost nobody makes, and it is the one built from the
assessor's own published figures. There is nothing for them to dispute except arithmetic they
published themselves.
Part 3 — How we find it
1We hold the county's own roll
Over five million Illinois parcels, with the assessed value the county published, the land component
where it is published separately, the class code, and the lot size computed from the county's own
GIS parcel polygon.
2We build a comparison group the assessor would recognise
Like-classified property in your own town — and in Cook, inside the assessor's own neighbourhood
code, which is the grouping they use to decide which properties are alike. Lot sizes within
60–170% of yours, because the relationship between lot size and value is not linear.
3We compare per square foot, against the median
Not against the cheapest neighbours we could find — that would be marking our own homework. The
benchmark is the median of the whole comparison group. The comparables we show are the ones
below it, because those are the evidence; the benchmark itself comes from all of them.
4We tell you if there is nothing there
Roughly a third of the properties we look at get told not to file. An assessment below the
neighbourhood median is a good position to be in, and an appeal invites a review that can go the
other way. A service paid out of your reduction cannot afford to tell you that. We are not.
A worked example
Your land assessment$14,810
Your lot10,119 sq ft
= your land, per square foot$1.46
Median for 400 comparable parcels, same class and town$0.81
You are above the median by81%
At the median your land would be assessed$8,176
So the request is your total, less that excess$75,126 (from $81,760)
That is a real parcel and a real result. Note what the ask is not: it is not $8,176. The
land argument reduces the total by the amount the land is over-assessed and by nothing else. Asking
a board for a 90% cut on evidence about a lot would discredit the whole filing.
Part 4 — What you do with it
You file it. Illinois boards of review accept an appeal from the owner or from a licensed
attorney, and from nobody else — so we prepare the document and the evidence, and you sign and
submit it. Filing is free; the board charges nothing to hear you.
The window is short: roughly 30 days from the publication of your township's assessment
roll (35 ILCS 200/16-55), and it is not the same date across a single county. A late filing
cannot be cured, which is why we will not produce a packet until you have confirmed the date against
your own notice.