A legal assessment for tax purposes on any parcel of property in any county in Illinois except Cook County is 33 1/3 percent of the property’s market value, excluding farmland and farm-related buildings. To determine whether your particular assessment is fair, you must know two factors: the market value of your property and the assessed value of your property. The market value of your property is defined as the price you would accept if a willing and able buyer would offer to purchase your property at today’s prices.
The assessed value of your property may be obtained from the township or multi-township assessor or from the county supervisor of assessments. Dividing the assessed value of your property by its market value will give you the percentage level of your assessment. By following this procedure for similar property in your area, you can compare your assessment to the average assessment level to determine if your property has been assessed at a fair value.

Your tax bill depends on two factors:
1) the assessment of your property and
2) the amount of money your local taxing districts need to operate during the upcoming year. The assessment on your property is set by local officials and is merely a method of fairly distributing the tax burden among all property owners in your community. The actual dollar amount of property tax you must pay is determined by the amount asked for by your local taxing bodies.
These taxing bodies annually determine the amount needed to pay for the services they provide. Individual tax bills are calculated to produce this total amount.

Any change in real estate assessments must be published in a newspaper of general circulation in each county every year. Every four years, when all property is reassessed, a complete list of assessments must be published for notification purposes. In addition, taxpayers outside Cook County must be mailed notice of any changes in their assessment from the previous year. Banks, savings and loans, or other mortgage holders are required to forward copies of all assessment change notices to affected borrowers within 15 days of the mortgage lender’s receipt of the notice.

If your property is outside Cook County, you can file a complaint with that County’s Board of Review. If you are dissatisfied with the decision of the board of review, you may appeal to the State Property Tax Appeal Board or pay under protest and take your complaint to the courts. However, court proceedings are generally more restrictive. A favorable court ruling is unlikely unless you have exhausted the administrative appeal process
Note: If you think your assessment is unfair it is important to file a complaint before you receive your bill. The "notice of assessment change" will explain the time period for filing a complaint.