COOK COUNTY, Ill. (WLS) — Hundreds of properties across Cook County are misclassified or undervalued, resulting in significant tax losses, according to an investigation by the Illinois Answers Project and the Chicago Tribune.

Last year alone, the Cook County Assessor’s Office overlooked new construction and major improvements on at least 620 properties. This resulted in at least $444 million in market value going untaxed in Cook County this year. These errors occurred despite there being documentation showing that these properties had been renovated or improved. Now, some homeowners are paying the price for this oversight.

Underassessment can hurt homeowners in at least two ways. Some people end up paying more than their fair share in property taxes, while others pay less. And when the assessor’s office corrects these errors, it can have drastic consequences on the property tax bill: The assessor can collect up to three years of back taxes in a single lump sum, which can amount to tens of thousands of dollars for a single homeowner.

You can read more about the Illinois Answers Project research here.