Rob Hartley

Founder, AppealDesk · February 16, 2026

Cook County Property Tax Appeal Guide (2026)

Updated February 2026 · 12 min read

The 10% Assessment Ratio: What It Means and How to Use It

Cook County assesses residential property at 10% of fair market value. This ratio is unique to Cook County — most other Illinois counties use 33.33%. The 10% ratio was established by the Cook County Classification Ordinance, which sets different rates for different property types (residential, commercial, industrial).

Here’s why the ratio matters for your appeal: you cannot compare your assessed value directly to home sale prices. You need to convert between the two using the 10% ratio.

This math is the foundation of every Cook County appeal. The IAAO considers assessment ratio analysis fundamental to fair property valuation. Many homeowners (and even some appeal services) skip this step and compare assessed values directly to sale prices, which produces nonsensical results in a 10%-ratio county. For a full breakdown of assessment ratios across all states, see our assessment ratio guide.

Cook County’s Triennial Reassessment System

Unlike most jurisdictions that reassess annually, Cook County reassesses properties every three years on a rotating schedule. The county is divided into three groups:

Reassessment years are the most important time to appeal because that’s when the Assessor assigns new values. Large increases typically happen during reassessment years. However, you can file an appeal in any year — not just your reassessment year.

For 2026, south and southwest suburban townships are being reassessed. If you live in this area, expect new assessment notices and pay close attention to the appeal window for your township.

How to File a Cook County Property Tax Appeal: Step by Step

Step 1: Check Your Township’s Appeal Window

Cook County uses rolling 30-day appeal windows by township. Check the Cook County Assessor’s website to see if your township is currently open for appeals. Set a calendar reminder — once the window closes, you must wait for the Board of Review period.

Step 2: Reverse-Engineer Your Implied Market Value

Take your assessed value and divide by 0.10 to see what the county thinks your home is worth. Then compare that implied market value to recent comparable sales. If comparable sales show a lower market value, you have a case.

Step 3: Gather Evidence

The Cook County Assessor’s Office accepts three types of evidence for residential appeals:

• Comparable sales: 3-5 recent arm’s-length sales of similar properties that sold for less than your implied market value. This is the strongest evidence type.
• Comparable assessments: Similar properties in your area with lower assessed values (per square foot or total). This argues for equalization — that your assessment should be consistent with neighbors.
• Property description errors: Incorrect square footage, room count, or property characteristics in the Assessor’s records. Check your property’s record card on the Assessor’s website.

Step 4: File with the Assessor’s Office

File online through the Cook County Assessor’s website. You’ll upload your comparable sales data and any supporting documentation. The Assessor’s office reviews your evidence and issues a decision — typically within a few weeks.

Step 5: Board of Review (Second Level)

If the Assessor denies your appeal or offers an insufficient reduction, you can appeal to the Cook County Board of Review. This is an independent three-member elected board that reviews appeals after the Assessor’s decision. The Board of Review has its own filing period — check their website for current dates. Many successful Cook County appeals are decided at this stage.

The State Equalizer: Why Cook County Taxes Feel Extra High

After your assessed value is set, the Illinois Department of Revenue applies an equalization factor (sometimes called the “multiplier”) to bring Cook County assessments in line with the state’s 33.33% target. For recent years, this multiplier has been approximately 2.9-3.2, effectively tripling your assessed value before tax rates are applied.

Here’s how the full calculation works for a $380,000 home:

The equalizer is set by the state and you can’t change it. But by reducing your assessed value through an appeal, you lower the base that the equalizer multiplies — which compounds the savings. A $7,000 reduction in assessed value translates to roughly $21,000 less in equalized value, saving approximately $1,365 per year at a 6.5% tax rate.

Common Cook County Assessment Errors

Cook County has faced scrutiny for assessment accuracy. Research from the Lincoln Institute of Land Policy and investigative reporting by the Chicago Tribune and ProPublica have documented systemic issues with assessment uniformity, particularly in lower-income areas. Common errors to check for:

• Incorrect square footage: The Assessor’s records may include finished basement space, enclosed porches, or outdated measurements. Verify against your actual living area.
• Wrong number of rooms: Extra bedrooms or bathrooms in the record inflate your assessment. Check your property’s record card online.
• Missing condition factors: The Assessor may not account for deferred maintenance, outdated systems (old HVAC, original plumbing), or structural issues.
• Neighborhood misclassification: Properties near commercial zones, highways, or industrial areas should be valued lower than interior residential locations.

Cook County Appeal Services

Several services specialize in Cook County appeals:

• AppealDesk: $49 flat fee for evidence packet with comparable sales, 10% ratio analysis, and Cook County filing instructions. You file yourself.
• Appeal.Tax (KSN Law): Full legal representation before the Assessor and Board of Review. Specializes in Cook County and collar counties.
• Ownwell: Full-service at 25-35% of savings. Covers Cook County through their Illinois operation.
• O’Connor: Full-service at a percentage of savings. One of the largest firms operating in Cook County.

For more options, see our best property tax appeal services comparison. For cost details, see how much a property tax appeal costs. Use our appeal checklist to stay organized.

5 Tips for a Successful Cook County Appeal

1. Always convert to implied market value. Don’t argue that your assessed value of $45,000 is too high by comparing it to a house that sold for $380,000. Convert: $45,000 ÷ 0.10 = $450,000 implied market value versus $380,000 actual market value.
2. Use both comparable sales AND comparable assessments. Cook County responds to equity arguments. If your neighbor’s similar home is assessed at $35,000 while yours is $45,000, that’s a compelling uniformity argument.
3. File at both levels. Appeal with the Assessor first, then appeal to the Board of Review regardless of the Assessor’s decision. The Board of Review frequently grants additional reductions.
4. Photograph condition issues. Dated kitchens, old windows, cracked driveways, and aging mechanicals all support a lower value. The Assessor’s mass appraisal assumes average condition unless you prove otherwise.
5. Check the record card. Verify every detail on your property’s record card: square footage, room count, age, building class, and lot size. Data errors are common and are the easiest type of appeal to win.

Frequently Asked Questions