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Cook County Property-Tax Basics November 11, 2026 7 min read

The Assessor re-checked 27,000 permits and flagged about a third. How Cook County's omitted-assessment bill works

Cook County can bill the current year and 3 prior years for an addition it never assessed. After a 27,000-permit audit: the bill, 7 defenses, how to contest.

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In late 2024 the Cook County Assessor audited 27,000 building permits in its system. About a third needed a second look, and the Assessor said it would field-inspect those properties during 2025 to bring missing value onto the tax rolls.

When the missing value is an addition, a new house or another improvement the county never assessed, the Assessor can issue an omitted assessment: a new value for the current year and up to three earlier years, followed by a bill for the tax those years should have carried.

State law also lists when the county cannot charge you for past years. Several of those turn on paperwork you may already have, starting with your building permit.

Quick Answer

An omitted assessment is the Assessor adding value it missed, usually new construction, an improvement that was never on the rolls, or a property that used to be tax-exempt. It can cover the current year and up to three prior years in which the tax went unpaid. You get a notice with an appeal deadline. The bill for past years arrives with a regular first-installment bill and is due with that year's second installment. You can appeal to the Assessor by the deadline on the notice, then to the Board of Review. The county cannot charge past years if, among other things, the Assessor already had a building permit showing the work, or the years came before you bought the home.

What triggers it

The Assessor's appeal rules say omitted assessments usually involve formerly tax-exempt property, new construction, or improvements left off the rolls. The Assessor finds them three ways:

  • Building permits. Towns and the city send permits to the Assessor, which sorts them and sends a field inspector when the work is likely to change the value.
  • Change detection from above. Since 2020 the Assessor has used a contractor that compares satellite and drone images. When no permit came in, a flagged change can send an inspector out.
  • Audits, such as the 27,000-permit audit in late 2024.

Before sending a notice, the Assessor's data collection team creates a new property record card for the home, and the omitted value comes from that card.

Owners have a duty too. State law says to tell the Assessor within 30 days of an occupancy permit or of finishing the work, and the Assessor has a New Construction Reporting Form for it. New work is taxed from the date it was ready to use, not just from the next January 1.

How far back, and what the bill looks like

  • Years covered: the current year and up to three prior years, only where the tax went unpaid.
  • Billing: the past-year bill is mailed with a regular first-installment bill and is due on that year's second-installment due date.
  • Late payment: 0.75% a month for omitted tax from tax year 2023 on, 1.5% a month for earlier years.
  • Interest on old years: state law adds 10% a year, starting two years after the correct bill should have arrived. It is waived when an incorrect survey or another clerical error by the assessor caused the omission and you paid your bills as they came.
  • Exemptions: the notice comes with homeowner and senior exemption applications for the years in question. If you lived there on January 1 of those years, send them back.

An illustrative example. Say an addition added $60,000 to a home's market value and never made the rolls. Cook assesses homes at 10% of market value: $6,000. The state multiplier for tax year 2024 was 3.0355, which makes about $18,213 of taxed value. At 8.52%, the median tax rate on Cook homes with the homeowner exemption in tax year 2024, that is about $1,552 a year, or about $6,200 for four years before interest. Real bills use each year's own multiplier and rates.

When the county cannot charge past years

State law lists seven situations in which no tax for past years can be charged. The Assessor's rules call them defenses and ask for documents to back them.

  1. The Assessor did not send its omitted-assessment decision to the Board of Review as the law requires.
  2. The property was last assessed as vacant land, you reported the new building and asked for a reassessment, and the Assessor did not act within 16 months.
  3. For formerly tax-exempt property, the new owner gave the required notice of the change.
  4. The Assessor received a building permit showing the new construction but did not put it on the rolls.
  5. The Assessor received a plat, survey, mortgage survey or similar document showing the improvement but did not list it.
  6. The Assessor received a sale record showing a sale from a tax-exempt owner to a taxable one but did not list the property.
  7. An earlier appeal at the Assessor or Board of Review already included the improvement, with evidence of its value.

One more rule protects buyers: no charges for years before you bought the home, if you paid for it in good faith without notice of the unpaid tax. In every case the current year can still be assessed.

How to contest it

Meet the deadline on the notice. The Assessor's rules say an omitted assessment must be appealed in the year it is issued. File through the Assessor's online appeal portal, following the instructions on its Omitted Assessment Appeals page. After the deadline, the only route is the Assessor's refund process (it calls it a Certificate of Error).

Use regular appeal evidence. Dated color photos, square footage, a plat of survey, an appraisal or comparable homes challenge the value. Permit copies, surveys and earlier appeal files support the defenses.

Then go to the Board of Review. The law requires at least 30 days' notice of the Assessor's hearing and a written decision that says it can be appealed to the Board. The Board says there is never a fee to file.

Contesting sometimes works. In the Assessor's public appeal records, owners of residential properties appealed omitted assessments on 109 parcels in tax years 2021 through 2025, and the Assessor changed the value on 54. It is a small group, and it does not show how many notices went unchallenged.

Check your own record against your permits now

  1. Pull up your property on the Assessor's site. Its property details list building square footage, bedrooms, baths, basement and garage. Our guide to property record card errors walks through each line.
  2. List your work and find the permits. Additions, finished basements, garages, dormers, and the year each was finished. Your town keeps permits, and the Assessor publishes the permits it has received as a public dataset on Cook County's open data site, searchable by PIN. A permit in that dataset is strong evidence the Assessor got it.
  3. Compare. If the record shows less than you built and the Assessor has no permit, that is what an omitted assessment is made for. The New Construction Reporting Form is how the law expects you to report it.
  4. If the record shows more than you have, that is an appeal point. See characteristic errors in the county's record.

Permits help another way. For an owner-occupied house, the Assessor applies the Home Improvement Exemption after it field-checks a permit: up to $75,000 of added value goes untaxed for up to four years, with no application.

Where Censum fits

Censum's free property check is built for the regular yearly appeal: it shows how appeals on homes like yours have fared and whether your township's window is open. An omitted-assessment notice runs on its own deadline, printed on the notice, so read that date first. Censum's figures start from the county's record of your home, one more reason to make sure that record is right. Our guide to your two appeal windows explains the Assessor and Board stages.

The numbers behind this article

The 27,000-permit audit is from the Assessor's March 17, 2025 news release. Appeal counts are Censum's tabulation of the Assessor's Appeals dataset on Cook County open data, accessed November 11, 2026: appeal type "omitted assessment," residential class 2 parcels, tax years 2021 through 2025 (109 parcels; 54 changed, 54 no change, 1 without a result). The public data labels this appeal type from tax year 2021 on. The 3.0355 multiplier and 8.52% median rate (among 1,005,437 PINs with a homeowner exemption, tax year 2024) are from PTAXSIM, the Cook County Assessor's Office research database, whose results the Assessor describes as not official. The worked example is illustrative.

FAQ

How many years back can an omitted assessment go?

The current year and up to three prior years, and only years in which the tax was not paid.

I bought the house with the addition already there. Can they bill me for the seller's years?

Not for years before you bought it, if you paid in good faith without notice of the unpaid tax. Years since you bought it can still be billed.

My contractor pulled a permit. Am I protected?

If the Assessor received the permit and still left the work off the rolls, the law bars charges for past years. The current year can still be assessed.

Can I appeal, and does it cost anything?

Yes. Appeal to the Assessor by the deadline on the notice, then to the Board of Review. Neither charges a fee to file.

Sources

Sources accessed November 11, 2026.

Next step

See how appeals on homes like yours have fared and when your township's window opens. Check my property

Censum LLC is a data and analytics company, not a law firm and not a tax advisor. Censum is independent and is not affiliated with Cook County or any government agency.