Buying below assessed value does not reduce your tax bill, and last year's bill is not a reliable guide to next year's. Both assumptions appear regularly in underwriting that later goes wrong.
How the cycle works
Cook County reassesses on a triennial cycle by township, so any given property is reassessed every three years rather than annually. A property assessed two years ago may be due.
Bills also arrive in arrears, which means the number you see at purchase reflects an earlier assessment, not the current one.
Why your purchase price matters
A recent sale is visible, and it is information the assessor can act on. Buying substantially below the assessed value does not automatically reduce the assessment, and a purchase at or above it can support an increase.
The point is not that a reassessment always follows a sale — it is that underwriting on the previous owner's bill assumes nothing changes, which is the one thing you should not assume.
Exemptions do not transfer
Homeowner and senior exemptions attach to the occupant, not the building. If you buy from an owner-occupant and rent the property out, those exemptions fall away and the bill rises accordingly.
This is one of the more common unpleasant surprises on single-family purchases in the suburbs.
What to do instead
Underwrite on what the bill is likely to become. Look at what comparable properties of similar assessed value are actually paying, and where the township sits in the reassessment cycle.
If the resulting number breaks the deal, that is useful to know before closing rather than at the first bill. Appeals exist and are worth pursuing, but a deal that only works if an appeal succeeds is a deal resting on an outcome you do not control.