Assessor

If you own property in Cook County, the Cook County IL Assessor has a direct impact on your property tax bill. Understanding what the Assessor does, how your home or business is valued, how to read your assessment notice, and when to appeal can make the difference between overpaying and paying a fair share. This guide walks through how the Assessor’s system works, how assessments tie into the tax bill you get from the Treasurer, how exemptions and Certificates of Error can save you money, and where to go for help if something looks wrong.

Understand What the Cook County Assessor Actually Does

The core job of the Cook County Assessor

The Cook County Assessor’s Office is responsible for estimating the fair market value of more than 1.8 million parcels in Cook County for property tax purposes. Its mission is to deliver accurate, transparent assessments for homes, condos, apartments, commercial buildings, industrial properties, and more.

Key points about the Assessor’s role:

The office estimates what your property would reasonably sell for in the open market (its “fair cash value”).

It then converts that market value into an assessed value, which is the portion of value used to calculate property taxes.

The Assessor does not set tax rates or decide how much money local governments collect; it determines each property’s share of the total tax burden.

To explore the office’s services, forms, news, and online tools, you can start at the official Cook County Assessor’s Office website.

Different assessment levels for different property types

Cook County uses classification levels set by county ordinance. Those levels determine what percentage of market value becomes assessed value:

Most residential property (1–6 units): assessed at 10% of fair market value

Most commercial, industrial, and many larger multifamily properties (7+ units): assessed at 25% of fair market value

Example for a typical home:

Fair market value: $300,000
Assessment level: 10%
Assessed value: $30,000

Example for a typical commercial building:

Fair market value: $1,000,000
Assessment level: 25%
Assessed value: $250,000

This difference in assessment levels is one reason residential and commercial owners can see very different tax outcomes even at the same market value.

The triennial reassessment cycle

Cook County is divided into three broad areas for assessment purposes:

City of Chicago
North suburbs
South and west suburbs

Each area is reassessed once every three years. In your reassessment year:

The Assessor updates your property’s market value as of January 1 of that year.

You receive a reassessment notice showing the new value, your property characteristics, and appeal deadlines.

Your property may also be reassessed between triennial cycles if there are significant changes, such as permits, demolition, or division of the property.

Look Up Your Cook County Assessment and Property Details Online

Use the address and PIN search tools

You don’t need to wait for paper mail to see what the Cook County IL Assessor has on file for your property. The office maintains an online property database where you can search by address, neighborhood, or your 14-digit Property Index Number (PIN).

To review your property’s current assessed value, basic characteristics, and other details, use the Assessor’s search by address tool.

When you look up your property, you can typically see:

Current and prior year assessed values
Estimated fair market value
Property classification code
Recorded characteristics such as building square footage, age, and type
Whether certain exemptions have been applied in recent years

If the characteristics are clearly wrong (e.g., the square footage is off, a demolished garage is still listed, or the property type is incorrect), that is a strong reason to consider filing an appeal.

See How the Cook County Assessor Values Homes and Condos

Mass appraisal and neighborhood sales

For most residential property, the Assessor uses mass appraisal, not individual in-person appraisals for each house. The office looks at:

Recent sales of homes near yours

Property characteristics such as:
Land area
Building square footage
Age and construction type
Location and neighborhood patterns
Features like porches or other improvements

Using a Computer Assisted Mass Appraisal (CAMA) system and statistical modeling, the Assessor detects how different characteristics affect sale prices across an area. Homes that are more similar to yours and closer to your location have more influence on your estimated value.

To dig into the official methodology and self-evaluation reports used for residential assessments, you can review the Assessor’s detailed explanation on the How Residential Property Is Valued page.

Why your purchase price and the Assessor’s value may differ

Many owners compare the Assessor’s estimate to what they recently paid for the property. Those numbers won’t always match exactly because:

Sale prices can be influenced by individual circumstances, timing, or negotiation.

The Assessor has to treat all properties fairly, including those that have not sold recently.

Models rely on several years of sales data to smooth out short-term swings in the market.

If the characteristics are accurate and the estimated market value is within roughly 10% of what you believe you could sell for, an appeal may not significantly change your tax bill. But if those numbers are far apart, an appeal is worth considering.

Learn How the Assessor Values Commercial and Larger Multifamily Property

The income approach for commercial property

For most commercial and industrial buildings, and many multifamily properties with seven or more units, the Cook County IL Assessor uses an income approach to estimate market value.

That process generally follows these steps:

Potential Gross Income (PGI)
What the property could earn annually if fully leased at typical market rents.

Vacancy and Collection Loss
A normal allowance for vacant space and unpaid rent, based on market data for similar properties.

Effective Gross Income (EGI)
PGI minus vacancy and collection loss.

Operating Expenses (including real estate taxes)
Typical expenses needed to operate the property, such as insurance, maintenance, management, and property taxes.

Net Operating Income (NOI)
EGI minus operating expenses. This reflects the net income the property generates.

Capitalization Rate (Cap Rate)
A market-derived rate that converts NOI into value. The Assessor uses an “unloaded” cap rate, meaning real estate taxes are treated as an expense rather than added into the cap rate.

Market Value Estimate
Market value = NOI ÷ cap rate.

Additional land value (if applicable)
If a property has more land than is typical for its type, extra land value may be added on.

After estimating market value, the Assessor applies the appropriate assessment level (generally 25% for commercial/industrial and 10% for many multifamily residential properties used primarily as housing).

Property owners can review this process in more depth, along with example calculations and methodology reports, on the Assessor’s How Commercial Properties Are Valued page.

Read Your Assessment Notice and Tax Bill Without Confusion

What the reassessment notice shows

Your reassessment notice is sent by the Assessor’s Office, usually in your triennial reassessment year. It includes:

Your Property Index Number (PIN)
Property characteristics
Estimated fair market value
Assessed value (after classification level)
A summary of exemptions applied in recent years
The last date you can file an appeal with the Assessor for that year

This notice does not demand payment. It is your chance to check whether the value and property characteristics look right and to appeal if they do not.

For a detailed walkthrough of every line on the notice and how it fits into the bigger tax picture, review the Assessor’s Your Assessment Notice and Tax Bill explanation.

How the assessment becomes your property tax bill

Your tax bill is sent by the Cook County Treasurer, not the Assessor. It comes in two installments:

First installment (due around March):
Equal to 55% of the prior year’s total taxes, by law.
Does not incorporate new assessments or changes yet.

Second installment (due in late summer):
Reflects your new assessment, any appeals, exemptions, and the latest tax rates.

The journey from assessment to tax bill typically follows this chain:

Fair Market Value – estimated by the Assessor.

Assessed Value – market value × assessment level (10% or 25%, in most cases).

Equalized Assessed Value (EAV) – assessed value × state equalization factor.

Adjusted EAV – EAV minus any applicable exemptions.

Tax Rate Applied – determined by local taxing bodies (schools, parks, libraries, etc.), calculated by the Cook County Clerk.

Final Tax Bill – adjusted EAV × local tax rate.

The Cook County Clerk’s Office can answer questions about your tax rate; the reference information notes a contact phone number for that office, which is listed at the end of this article.

Use Exemptions to Cut Your Cook County Property Tax Bill

Common exemptions for homeowners and families

Exemptions reduce the taxable value (EAV) used to compute your bill. They do not change the Assessor’s estimated market value, but they can significantly lower the amount you pay.

Major exemptions administered by the Assessor’s Office include:

Homeowner Exemption – for primary residences occupied by the owner.

Senior Exemption – for qualifying senior homeowners.

“Senior Freeze” Exemption – for eligible seniors with income limits, designed to help protect against tax increases from rising assessments.

Persons with Disabilities Exemption – for qualifying individuals with disabilities.

Veterans with Disabilities Exemption – for eligible veterans with service-connected disabilities.

WWII Veterans and Returning Veterans Exemptions – for eligible veterans meeting specific criteria.

Longtime Homeowner Exemption – for qualified long-term owner-occupants meeting income and occupancy requirements.

Home Improvement Exemption – for certain qualifying improvements that temporarily limit increases due to new work.

To see full eligibility rules and to file online where available, visit the Assessor’s Exemptions overview.

Checking for missing exemptions

The Assessor encourages homeowners to review the lower left corner of their tax bill to confirm that all expected exemptions are listed. If one is missing:

You may be able to apply for a Certificate of Error to receive a correction and potential refund for prior years, subject to current processing timelines and legal limits.

As of the latest notices on the Assessor’s site, valuation-related Certificates of Error have processing and refund timing constraints, and the office periodically updates those details in online alerts.

You can learn about eligibility and apply online through the Assessor’s Certificates of Error application page.

See How the Cook County Assessor Uses Data to Value All Properties

The Assessor’s Office uses mass appraisal to promote fairness across Cook County. Rather than sending staff to individually appraise every property each year, the office:

Divides the county into areas and townships.

Collects several years of sales data.

Builds statistical models that connect sale prices to property characteristics.

Tests hundreds of models against actual sales to meet professional standards for accuracy.

Applies the best-performing models to estimate values for all properties in a given area.

Has analysts review results neighborhood by neighborhood and adjust individual assessments where needed.

This approach is designed to treat similar properties similarly and to make the assessment system more consistent and transparent, even in a large jurisdiction like Cook County.

Know When and How to Appeal Your Cook County Assessment

When you should consider appealing

You should think seriously about filing an appeal with the Cook County IL Assessor if:

The property characteristics listed (square footage, building type, condition, or improvements) are materially wrong.

The estimated market value is significantly higher than what you believe your property could sell for in the current market.

Your property has issues that the Assessor’s model may not fully capture, such as unusual conditions, chronic vacancy, or damage.

The Assessor’s own guidance includes a practical rule of thumb:

If characteristics are correct and the estimated market value is within about 10% of what you believe your home is worth, an appeal might not lead to a large enough change to substantially affect your tax bill.

Appeals are free to file, and you do not need to hire an attorney or tax representative to submit one.

Appeal deadlines and the yearly schedule

Appeals can only be filed during specific windows:

In your reassessment year, you typically have about 30 days from the date your reassessment notice is mailed to appeal with the Assessor.

If you miss that window, you may be able to appeal the following year when your township opens again for appeals, or appeal later with the Cook County Board of Review.

To see when your township’s appeals window opens and closes, use the Assessor’s Assessment & Appeal Calendar.

How to file an appeal with the Assessor

The Assessor encourages property owners to file online. The process generally includes:

Look up your property and confirm your PIN.

Gather evidence such as comparable property information, recent sale data, or documentation of errors in property characteristics.

Complete the online form, selecting the correct property type (residential, commercial, industrial, vacant, not-for-profit, etc.).

Upload supporting documents and submit before the deadline listed for your township.

You can start the process through the Assessor’s online appeals filing portal. All filers must follow the official appeal rules posted by the office.

Once you’ve filed an appeal in your reassessment year, you do not have to file again every single year unless something about your property or its condition has changed.

Connect the Assessment to Your Bottom-Line Tax Bill

How exemptions and appeals show up on the bill

Changes to your assessment and exemptions will not appear on your tax bill immediately:

A reassessment done in one tax year affects the second installment bill in the following year.

If your appeal is granted and your assessed value is reduced, you should see that reduction reflected on that later second installment bill.

Exemptions that are approved will reduce the Equalized Assessed Value (EAV), which then reduces the final tax amount once the local tax rate is applied.

The Assessor’s reference materials include example calculations showing how:

An EAV reduction from exemptions is multiplied by your local tax rate to determine savings.

The equalizer and classification system work together to arrive at your taxable value.

If your bill does not match what you expected after an appeal or exemption approval, it can be helpful to:

Compare the EAV, exemptions, and rate listed on the bill to your most recent assessment notice.

Contact the Assessor for questions about assessed value or exemptions.

Contact the Treasurer’s Office for questions about payment status or billing, and the Clerk’s Office for questions about tax rates.

Get Help from the Cook County Assessor’s Office and Local Township Assessors

When to reach out directly

If you’re unsure whether your assessment is fair, or you need guidance with forms, the Assessor’s team provides several ways to get help:

Online information pages on valuation, appeals, exemptions, and the property tax system.
Contact forms and phone support for general questions.
In-person assistance by appointment at downtown and branch office locations.

The office periodically posts important alerts online, such as:

Temporary delays in processing Certificates of Error related to valuation and refunds.

The remodeling-related closure of the Assessor’s Markham Branch Office located inside the Markham Courthouse, along with instructions to book appointments at nearby branches instead.

Work with your local township assessor

For many suburban owners, local township assessor offices can help you:

Understand your assessment and property characteristics.
Gather comparable property information.
Decide whether to appeal with the county Assessor or Board of Review.

To find contact information for your township assessor’s office, use the countywide Local Township Assessors list.

Departments and Offices Related to the Cook County Assessor

Cook County Assessor’s Office – 118 North Clark Street, Third Floor, Room #320, Chicago, IL 60602 – Phone: (312) 443-7550

Cook County Clerk’s Office – Phone: (312) 603-6566

Cook County Assessor FAQs

How does Cook County calculate the market value that appears on my notice?

For homes, the Assessor’s Office relies on “mass appraisal,” using recent sales of similar properties in your neighborhood and your home’s characteristics—things like square footage, age, land, construction type, and location—to estimate fair cash value. Advanced statistical models (CAMA) test hundreds of approaches to see which best matches real sale prices before values are assigned and reviewed by analysts township-by-township. You can dig into the official modeling details on the Assessor’s page How Residential Property Is Valued.

How often will my property be reassessed, and when should I expect a notice?

Cook County reassesses property on a three-year cycle, rotating between the City of Chicago, the northern suburbs, and the southern suburbs. Your home is valued as of January 1 using three to five years of prior sales data, and you receive a reassessment notice in the year your area is scheduled. Properties can also be re-reviewed mid-cycle if there are major changes like permits, demolitions, or divisions. For timing and how that connects to your tax bills, see Your Assessment Notice and Tax Bill.

When is it worth filing an assessment appeal, and where do I start?

An appeal is most useful if the notice has incorrect characteristics (for example, wrong square footage or missing damage) or if the estimated market value is clearly higher than what your property could sell for. As a rule of thumb, if the value is within about 10% of what you think is realistic, a change may not move your tax bill much. Appeals are free, you don’t have to hire anyone, and you typically have around 30 days from the mailing date. Start with the Assessor’s Overview of How Appeals Work and check filing windows for your township on the assessment and appeal calendar.

If my assessment changes, will my property tax bill automatically change the same amount?

Not one-to-one. Your estimated market value is first converted to an assessed value (10% for most residential, 25% for most commercial), then adjusted by the state equalizer and reduced by any exemptions before local tax rates and levies are applied. Because those levies and the total value of all property in your community can shift, your assessment can rise while your share of the tax burden stays flat or even drops. The impact of reassessment and appeals shows up on the second-installment bill in the following year; the mechanics are broken down in Your Assessment Notice and Tax Bill.

How can I make sure I’m getting every exemption or refund I qualify for?

Homeowners are urged to look at the exemptions section on the tax bill—if a homeowner, senior, disability, or veterans exemption you should receive is missing, you can submit an online application or schedule office help to correct it. In some cases, a Certificate of Error can retroactively fix past years’ assessments so tax savings can be applied. The Assessor’s Exemptions pages explain each program, who qualifies, and how to file, and the Certificates of Error information linked there outlines how to track the status of any correction that has already been submitted.