Assessed Value vs. Market Value: Why They're Different
If your Zillow estimate says $700,000 but your tax bill shows an assessed value of $47,000, you're not misreading anything. They're measuring two completely different things.
What is market value?
Market value — the number Zillow shows as a “Zestimate” — is the price your home would likely sell for today on the open market. It reflects current buyer demand, recent sales of comparable homes, interest rates, and local conditions. It moves constantly and can swing dramatically year to year.
Market value is not used directly to calculate your tax bill in Cook County.
What is assessed value?
Assessed value is a number assigned by the Cook County Assessor's Office (CCAO). For residential properties (Class 2), the CCAO is supposed to assess your home at 10% of its estimated market value. So a $700,000 home should have an assessed value around $70,000.
In practice, the CCAO's estimate of your home's market value may be far from accurate — especially if your neighborhood hasn't been visited in years or if the underlying sales data used is thin.
How the math works
Rates vary by township and tax year. This example uses illustrative figures.
Why the Zillow number and the assessed value look so different
If Zillow shows $715,000 and your assessed value is $47,000, the math works out: $47,000 ÷ 10% = $470,000 — that's the CCAO's estimate of your home's market value. The difference between $470,000 and $715,000 means the Assessor thinks your home is worth less than Zillow does.
This doesn't necessarily mean you're over-assessed — it just means the two estimates differ. Over-assessment is determined by comparing your assessment relative to your neighbors, not relative to Zillow.
What actually drives over-assessment
The Illinois Property Tax Code requires that similar properties be taxed uniformly. If your neighbor's home — same size, same age, same neighborhood — is assessed at $8/sqft while yours is at $12/sqft, you're paying 50% more per square foot than they are. That's not a market value question — it's a fairness question, and it's exactly what a uniformity appeal addresses.