Illinois Property Tax Appeal | 2026 Guide

A Certificate of Error (CofE) is a special Illinois procedure that lets the Cook County Assessor fix a mistake on a property tax bill after it has already been issued — outside the normal township appeal window. The Assessor describes it as the process "to allow the Cook County Assessor to apply changes to a property tax bill that has already been issued." See Certificates of Error.

Two main uses. The procedure addresses (1) missing homeowner exemptions for prior tax years and (2) corrections to a property's assessed valuation after a bill has issued. The most common use for homeowners is recovering exemptions (such as the general Homeowner Exemption or Senior Exemption) they qualified for but did not receive.

How far back it reaches. For missing exemptions you can currently apply for tax years 2024, 2023, 2022, and 2021 — roughly four prior years. This is a powerful remedy if you only recently discovered you were overpaying because an exemption was never applied.

The process and refund. You apply online or by paper form. The Assessor's office processes applications in about 8–10 weeks and notifies you of approval or denial. If approved, the Cook County Treasurer's Office issues a refund check by mail, typically within 3–4 weeks of approval.

When to use it instead of an appeal. Use a Certificate of Error when (a) you missed the appeal window but have an objective error, (b) a qualifying exemption was never applied, or (c) the property record contains a factual mistake the office can verify. For a current-year valuation dispute where your township window is still open, file a normal appeal with the Assessor and, if needed, the Board of Review. The CofE is the catch-up tool for errors a regular appeal can no longer reach.

Whether your home is in the City of Chicago or a Cook County suburb, the appeal structure is the same: you appeal first to the Cook County Assessor, then optionally to the separate Cook County Board of Review, and finally to the state Property Tax Appeal Board if needed. The 10% residential assessment level, the PIN system, and the township-window mechanics apply countywide.

The real difference is timing, set by the triennial cycle. Cook County's reassessments rotate across three groups:

• City of Chicago townships,
• north and west suburbs, and
• south and west suburbs.

Each group is reassessed in a different year of the three-year cycle, so the year your area gets a fresh value — and the year your appeal window is most active — depends on which group your township is in. In 2026 the south and west suburbs are being reassessed, with notices mailing in late April; Chicago and the north suburbs are not on the regular 2026 reassessment and are only re-valued mid-cycle for changes like new construction or permits. See Learn about Reassessments.

What this means for you.

• Chicago homeowners should track the City's reassessment year and their specific township's window — Chicago is divided into townships for assessment, so the same township-by-township calendar applies.
• Suburban homeowners in a current reassessment group will receive a notice that opens their window; those outside the current group can still appeal during their township's annual window even without a reassessment.
• In every case, confirm your township and its open/close dates on the Assessment & Appeal Calendar rather than assuming a city-wide or suburb-wide date.

Bottom line: same process, same offices, same rules — only the calendar shifts between Chicago and the suburbs because of where each falls in the triennial reassessment rotation.

In Cook County, every parcel carries a class code that describes what kind of property it is, and the code drives how the Assessor values it. Residential property falls under Class 2, but within Class 2 there are many subclasses based on the building's stories, square footage, age, and use. See Classifications of Real Property.

Examples of Class 2 subclasses.

• Class 2-03 — a one-story single-family residence of roughly 1,000 to 1,800 square feet.
• Class 2-11 — a building with two to six residential apartments, any age.

If your single-family home is mistakenly coded as a multi-unit building (or vice versa), the Assessor may be valuing it on the wrong basis and the record may carry wrong characteristics. A classification appeal asks the office to correct the code.

When to file one. Consider a classification appeal if:

• Your property's recorded class does not match how it is actually used (e.g., it is coded 2-11 but is a single-family home).
• The square footage, story count, or unit count in the record is wrong, pushing you into the wrong subclass.
• A prior division, conversion, or renovation changed the building but the class was never updated.

Evidence the Assessor expects. Under the Official Appeal Rules, class-change requests must document the building accurately. For a change to a multi-unit class like 2-11, the rules call for the total number of rooms broken down by unit, plus evidence each unit is genuinely residential — a separate entrance, separate mailbox, and separate utility meters — and they recommend current leases and rent rolls where applicable. For a single-family code, provide proof of single-family use, square footage, and story count.

File it within your township window. A classification correction is filed as part of your assessment appeal during your township's open window through the Assessor's appeals system. Confirm your property's current class by PIN before filing so you know exactly what you are asking to correct.

Unlike most jurisdictions that use one annual deadline, Cook County runs assessment appeals on a rolling township schedule. The county is divided into 38 townships, and the Cook County Assessor opens each township's appeal window separately over the course of the year. You cannot file until your township is officially open, and you must file before it closes.

How long the window stays open. The Assessor's office advises that taxpayers "typically have 30 days to file an appeal after receiving your reassessment notice. The last date to file an appeal for that year is printed on your notice." Confirm both the open and close dates on the Assessment & Appeal Calendar, which lists every township's status. See the Assessor's Appeals overview for the full process.

Filing is free and online. The Assessor states that "filing an appeal is free and can be done online in as little as 20 minutes" through the online appeals system; paper appeals are available for owners without online access. All appeals must follow the Official Appeal Rules of the Cook County Assessor.

You may not need to file every year. Because Cook County reassesses on a triennial cycle, the Assessor notes you generally file in a reassessment year and "do not need to do it each year, unless the characteristics of your property have changed significantly due to new construction, demolition, vacancy, or other issues." That said, you may file annually if you believe the value is too high.

The Assessor is only the first stop. If the Assessor denies or partially grants your appeal, you can separately appeal to the Cook County Board of Review, which runs its own township windows. Watch your township's status closely — once a window closes, you lose that stage for the year.

The Cook County Board of Review (BOR) is an independent body, not part of the Assessor's office. It describes itself as "a government office that provides the taxpayers of Cook County an opportunity to appeal the over-valuation of property assessments." It is the second administrative stage in the Cook County appeal sequence.

Separate township windows, opening after the Assessor's. Like the Assessor, the BOR processes appeals on a township calendar, but its windows open after the Assessor has closed and certified that township. Each township has its own open and close dates — there is no single county-wide BOR deadline. Confirm your township's window on the BOR's Dates and Deadlines page and its township open/close schedule.

You can go straight to the BOR. You do not have to appeal to the Assessor first. Many homeowners appeal to the Assessor, and if unsatisfied with the result, appeal again to the BOR; others skip the Assessor and file only with the BOR. Each is a fresh review of your assessment.

Filing online. Submit an appeal through the BOR's online appeals portal. The portal walks property owners through entering their Property Index Number (PIN), selecting an appeal type, and uploading evidence such as comparable assessments or recent sales.

After the BOR. The BOR issues a written decision and mails it to you. If you are still dissatisfied, you may appeal the BOR's decision to the state-level Illinois Property Tax Appeal Board within 30 days of the decision under 35 ILCS 200/16-160, or pursue a tax-objection complaint in the circuit court. The BOR stage is required before you can reach the Property Tax Appeal Board.

Illinois has 102 counties, and Cook is the only one with a separate two-stage Assessor-then-Board-of-Review system. In the collar counties — DuPage, Lake, Will, Kane, McHenry — and the rest of the state, the appeal process is simpler: you appeal to the county Board of Review (BOR), which is the primary administrative venue.

The deadline is publication-based, not notice-based. In DuPage County, for example, "the period during which an appeal may be filed ends thirty days after the publication of the township assessment roll." Assessment changes are published in local newspapers, and the 30-day appeal window runs from that publication date, not from when you personally receive a change-of-assessment notice. Confirm your township's publication date and BOR deadline on the DuPage County Board of Review page; Lake, Will, and other counties publish equivalents on their own supervisor-of-assessments sites.

Who can file and how. The BOR accepts a complaint filed by the owner, the taxpayer, or an Illinois-licensed attorney with proper authorization. The county forwards your complaint to the township assessor, who prepares a response presented at the BOR hearing. Most collar counties accept online or paper filing through their supervisor-of-assessments office.

Escalation is the same statewide. A collar-county BOR decision is appealable to the state-level Property Tax Appeal Board (PTAB) within 30 days of the decision notice under 35 ILCS 200/16-160, or you may pay under protest and file a tax-objection complaint in the circuit court. The 35 ILCS 200 Property Tax Code framework — including the 10% residential level applied through each county's own classification practice and the state equalization factor — applies across Illinois, but the first stop is your county Board of Review, and its deadline is the one you must not miss.

Practical tip: because collar-county deadlines key off township-roll publication, watch the legal-notices section of your local paper and your county's assessment-status page rather than waiting for mail.

Illinois requires that property across the state be assessed at a uniform level (one-third of fair market value statewide). Because Cook County assesses residential property at only 10% of market value under its own classification ordinance, the Illinois Department of Revenue applies an equalization factor — the "multiplier" or "equalizer" — to raise Cook County assessments to the statewide standard.

The formula. Equalized Assessed Value (EAV) = Assessed Value × Equalizer. The Assessor explains: "with an Equalizer of 3.0, a home with an assessed value of $50,000 would have an EAV of 3.0 × $50,000 = $150,000." The EAV — not the raw assessed value — is what local exemptions are subtracted from and what the tax rate is applied to. See the Assessor's Your Assessment Notice and Tax Bill.

The current figure. The Illinois Department of Revenue announced a final 2024 Cook County equalization factor of 3.0355 (announced May 29, 2025). The Department sets the multiplier by comparing actual selling prices to assessed values over a three-year period; the three-year median level of assessments for Cook County was about 10.98%. See the Department's 2024 Cook County Final Multiplier announcement.

Why you cannot appeal the multiplier. The equalizer is set county-wide by the state and applies identically to every property, so it is not something an individual homeowner can challenge in an assessment appeal. What you can challenge is your underlying assessed value — the 10%-of-market-value figure the Assessor assigned. Lowering that number flows straight through the multiplier and reduces your EAV and ultimately your tax. Focus your appeal evidence on market value and on unequal treatment relative to comparable homes, not on the equalizer.

Illinois gives a taxpayer who is unhappy with a county Board of Review (BOR) decision two mutually exclusive escalation routes for that tax year:

1. Appeal to the state Property Tax Appeal Board (PTAB) within 30 days of the BOR decision under 35 ILCS 200/16-160; or 2. Pay the tax under protest and file a tax-objection complaint in the circuit court under 35 ILCS 200/23-5 through 23-30.

How the court route works. Under the tax-objection procedure (35 ILCS 200/23-5 et seq.), you pay your property tax bill, marking the disputed portion as paid under protest, and then file a complaint in the circuit court of the county. The court reviews whether the assessment was excessive or otherwise improper. If you win, you receive a refund of the over-paid amount.

You cannot use both routes for the same year. Choosing PTAB and choosing the circuit-court tax objection are alternatives — pursue one or the other for a given assessment year. For most homeowners, PTAB is the more practical path: it charges no filing fee for residential appeals and does not require paying the disputed tax up front or hiring a litigator. The court route is generally chosen for larger disputes, or where a legal (not just valuation) question is involved, and typically involves attorney's fees and litigation timelines.

Timing. The court route is tied to your tax-payment deadlines rather than the 30-day PTAB clock, so confirm the exact filing window with the circuit court clerk or your attorney. Because the two routes diverge after the BOR decision, decide early which one fits your situation, and preserve the BOR decision notice — both routes require it.

Bottom line: the pay-under-protest court complaint is a legitimate alternative to PTAB, governed by 35 ILCS 200/23-5 through 23-30, but it is an either/or choice and usually the costlier of the two.

The Property Index Number (PIN) is the 14-digit identifier Cook County assigns to your parcel. Every assessment appeal — to the Assessor, the Board of Review, or the state Property Tax Appeal Board — is keyed to your PIN, so you need it before you can file.

Where to find it. Your PIN is printed on:

• your property tax bill from the Cook County Treasurer,
• your closing documents and deed, and
• notices from the Assessor's office, including your reassessment notice.

See the Assessor's guide, Where do I find my PIN?.

Look it up by address. If you cannot locate a document, use the Assessor's Search by Address tool. Enter your address to retrieve your PIN along with your estimated market value, assessed value, and property characteristics. You can also reach the same data through the Assessor's property details tool, which shows permits, divisions and consolidations, and exemption history.

Use the PIN to build your appeal. Once you have your PIN you can:

• Confirm the assessed value the Assessor assigned (multiply by 10 for the implied fair market value under the 10% residential level).
• Check the property characteristics on record — square footage, room count, building class — for errors you can challenge.
• Pull Neighborhood Sales for comparable transactions within the last 36 months, useful evidence that your home is over-assessed.

If your property has multiple PINs (for example, a parcel that was divided or a condo with separately numbered units), confirm which PIN matches the assessment you are appealing — filing under the wrong PIN can invalidate your appeal.

Filing an assessment appeal in Cook County is free, but the online systems require you to set up an account first so the office can track your submission and contact you. There is no fee and no separate qualification step — "pre-registration" simply means creating a login before you upload your appeal.

Cook County Assessor. The Assessor states that "filing an appeal is free and can be done online in as little as 20 minutes." You begin at the online appeals system, where you create or log into an account, look up your property by Property Index Number (PIN), select your appeal reason, and attach evidence. Owners without internet access can file on paper instead. All filings must follow the Official Appeal Rules of the Cook County Assessor.

Cook County Board of Review. The BOR runs a separate online appeals portal with its own account login. You register, enter your PIN, choose an appeal type, and upload supporting documents such as comparable assessments or a recent sale.

Key practical points.

• Create your account before your township window closes — do not wait until the final day, when the portal may be busy.
• You can only file when your township is officially open; an account alone does not let you file early.
• Keep your confirmation number after submitting; it is your proof of timely filing.
• An account at the Assessor does not carry over to the Board of Review — you set up each separately.

If you hire an attorney or authorized representative, they file through these same systems on your behalf under the office's appeal rules.

The Property Tax Appeal Board (PTAB) is a state agency that reviews local property assessment decisions. It is the next step after the county Board of Review (BOR) — in Cook County and in every other Illinois county — for homeowners who believe the BOR's decision still leaves them over-assessed. See the Illinois Property Tax Appeal Board.

You must go through the Board of Review first. PTAB does not hear an appeal until a county Board of Review has issued a decision. Under 35 ILCS 200/16-160, any taxpayer dissatisfied with the BOR's decision may appeal to PTAB. The statute distinguishes counties of 3,000,000 or more inhabitants (Cook County) from smaller counties, but the core right — and the deadline — applies statewide.

The 30-day deadline. You must file the PTAB appeal within 30 days after the date of the Board of Review's written notice of decision. This deadline is strict, so calendar it from the date printed on your BOR decision, not the date you received it.

No filing fee. Filing a residential appeal with PTAB does not require a fee. PTAB conducts a de novo review — it looks at the evidence fresh rather than merely auditing the BOR — and bases its decision primarily on equity (uniformity with comparable properties) and market value.

Your alternative: pay under protest and go to court. Instead of PTAB, a dissatisfied taxpayer may pay the tax under protest and file a tax-objection complaint in the circuit court under 35 ILCS 200/23-5 through 23-30. You choose one route or the other for a given year, not both. For most homeowners PTAB is the lower-cost path because it requires no fee and no attorney.

Practical tip: bring the same evidence that supported your BOR appeal — comparable assessments and recent sales — and emphasize uniformity, since PTAB places heavy weight on whether your assessment is consistent with similar properties.

Cook County is divided into 38 townships, and the township a property sits in controls the timing of its appeals. Both the Cook County Assessor and the Board of Review open and close appeal windows one township at a time, so you cannot find your deadline until you know your township. See the Assessor's guide, How do I look up my property's township?.

How to find your township. Look it up by Property Index Number (PIN) or address using the Assessor's Search by Address tool. Your property's detail page lists its township along with assessed value, class, and characteristics. The City of Chicago is itself divided into townships for assessment purposes, so even Chicago properties have a township assignment.

Why it matters.

• Your deadline depends on it. The Assessment & Appeal Calendar lists open and close dates by township; a deadline that applies to one township does not apply to a neighboring one.
• Your reassessment year depends on it. Townships are grouped into the three triennial reassessment groups (City of Chicago, north/west suburbs, south/west suburbs); your township tells you which year you are reassessed.
• Comparable evidence is township-aware. When you build an unequal-assessment argument, the Assessor and Board of Review expect comparables drawn from the same neighborhood and class within your area.

Practical workflow. (1) Find your PIN. (2) Look up your township. (3) Check that township's open/close dates on the Assessor's calendar and, later, on the Board of Review's deadlines. (4) File within the window. Skipping straight to a deadline page without confirming your township is the most common reason homeowners miss their filing window.

Cook County uses a triennial reassessment cycle: each property is reassessed once every three years rather than annually. The Cook County Assessor's office alternates reassessments across three groups of townships — the City of Chicago, the north and west suburbs, and the south and west suburbs — so one of the three is reassessed each year. See Learn about Reassessments and Your Assessment Notice and Tax Bill.

2026 reassessment group. In 2026 the south and west suburbs are being reassessed. Property owners in that group will be mailed a Reassessment Notice — containing the property address, characteristics, and updated estimated fair market value — beginning in late April 2026. Properties in the City of Chicago and the north suburbs are not on the regular 2026 reassessment, and are only reassessed mid-cycle if there is a change due to division work, permit applications, or other special circumstances.

Why the reassessment year matters for appeals. Your strongest, simplest time to appeal is the year your township is reassessed, because that is when the Assessor sets a fresh value and mails a notice that opens your appeal window. The Assessor notes that once you successfully appeal in a reassessment year, you generally "do not need to do it each year, unless the characteristics of your property have changed significantly."

You can still appeal in off years. Even when your area is not being reassessed, you may file an appeal during your township's annual window if you believe the value is too high — for example, if comparable homes are assessed lower or the record overstates your square footage. Check your township's reassessment status and current appeal window on the Assessment & Appeal Calendar.