An assessment is one input to a tax bill
An assessed value, an estimated market value and a tax bill answer different questions. The Assessor describes residential assessed value as 10% of fair market value. The eventual bill also depends on equalization, exemptions and the rates of the relevant taxing districts. A percentage change in assessed value is therefore not a promised percentage change in taxes.
Start by identifying the PIN and tax year on the official notice. A street address may describe several condominium or building parcels; use the parcel number to make sure the notice and comparison concern the same record.
Sources: Assessor: how residential property is valuedAssessor: how properties are valued
Compare the same year and stage
Cook County Records labels the assessment year alongside the recorded total. Its reconstruction uses the most final available stage for a year: Board, certified, then mailed. A later release may contain a more final value for the same year. Keep the source publication date with the number so a later reader can reproduce your comparison.
Check land and improvement amounts as well as the total. If a home’s recorded size, class or construction year looks inconsistent with the property, verify the official characteristics before using a calculated comparison. The site’s market-value and annual-tax figures are labeled estimates, not an appraisal or a tax calculation issued by the county.
Build a traceable research note
Record the subject PIN, assessment year and stage, source date, property characteristics and the reason for comparing particular neighbors. Link the official source and the parcel page. A high peer gap identifies a question to investigate; it does not prove the assessment is incorrect or that an appeal will succeed. Check the current official appeal calendar before acting.
Sources: Assessor: overview of appeals