Assessment Ratio
10% (Cook) / 33% (other)
Appeal Deadline
Varies by township (check your county, 2026)
Official Form
BOR Complaint
Risk of Increase
Yes
How Assessment Works in Illinois
Illinois uses a fractional assessment system. Cook County (Chicago) assesses residential property at 10% of market value, while most other counties assess at 33.33%. A state equalization factor ("multiplier") is applied to bring assessments closer to the statutory level. Your assessed value should equal the correct percentage of fair market value.
Risk of Increase
In Illinois, filing a appeal can result in your assessed value being increased. This is uncommon when you have strong evidence of over-assessment, but you should be aware of this possibility. ProtestMax only recommends filing when our analysis shows a clear case for reduction.
How to File in Illinois
In Cook County, file online through the Cook County Assessor's website during the open appeal period for your township. In other counties, file with the county Board of Review.
Step-by-Step
Illinois Appeal Process
Receive your assessment notice showing your property's assessed value.
Check the appeal period for your township (Cook County reassesses in triennial cycles).
File a complaint with the Board of Review (or Cook County Assessor for the first-level appeal).
Submit comparable sales evidence and any property condition documentation.
If the Board of Review denies your appeal, you can appeal to the Illinois Property Tax Appeal Board (PTAB) or circuit court.
FAQ
Illinois Property Tax Appeal Questions
What is the Cook County equalization factor?
The state equalization factor (or "multiplier") is applied to Cook County assessments to bring them to the statutory 33.33% level. For example, if the multiplier is 2.9, a home assessed at $30,000 has an equalized value of $87,000. The multiplier changes annually.
When is the appeal deadline for Cook County?
Cook County appeals open on a township-by-township basis during the triennial reassessment cycle. Each township has a 30-day window. Check the Cook County Assessor's website for your township's specific dates.
Can my Illinois assessment increase on appeal?
Yes. The Board of Review has the authority to increase your assessment. However, increases are rare when you present strong evidence of over-assessment. Most appeals result in reductions or no change.