Do Property Tax Appeals Actually Work? What the Data Shows
Yes — challenging an assessment measurably works often enough to be worth the effort, and we can prove it with real outcomes instead of a marketing number. In Cook County, Illinois, we tracked every residential parcel in the county's own assessment data and found that 189,430 of them — about 1 in 8 — ended 2025 with a lower assessed value than the county had certified, removing roughly $639 million in assessed value from the tax rolls.
That's not an estimate pulled from a survey or a law firm's client testimonials. It's computed directly from the Cook County Assessor's published parcel-level records. We're publishing the full methodology and the year-by-year numbers back to 2019, because as far as we can tell, almost nobody else has done this at this scale with real government data. Most of what circulates online under the banner of "property tax appeal success rate" is unsourced.
"Success rate" is a slippery number
Search "property tax appeal success rate" and you'll find confident-sounding figures like "70% of appeals succeed" or "homeowners win 60% of the time," almost none of them linked to an actual dataset.
The reason these numbers are so easy to produce, and so hard to trust, is that the same appeal process can generate wildly different "success rates" depending on which parcels get counted.
Three different denominators, three different answers
- All parcels. The broadest, most conservative measure — every residential property in the county, whether or not the owner did anything. This is the number we report below, and it's necessarily the lowest of the three.
- Filed appeals only. A much smaller, self-selected group. People who file generally believe they have a case, so this rate is meaningfully higher than the parcel-wide rate.
- Heard appeals only. Smaller still, and the most flattering number, because withdrawn, incomplete, or clearly weak filings have already dropped out before the count starts.
Why the denominator matters
Most published "success rates" quietly pick the third option — heard appeals — because it produces the biggest, most shareable number. That's not necessarily dishonest, but without saying which denominator is in play, the figure is close to meaningless for a homeowner deciding whether filing is worth their time.
We think the honest way to answer "do appeals work" starts with the broadest, most verifiable denominator available. That's what our Cook County research does.
What we found: the Cook County numbers
We used the Cook County Assessor's Assessed Values dataset (Cook County Open Data, resource uzyt-m557), filtered to residential (class 2xx) parcels, and compared each parcel's certified assessed value against where it ended the year. The full breakdown, methodology, and year-by-year figures are published here:
Cook County Property Tax Appeal Outcomes — full research and methodology →
The headline figures for 2025:
- 1,587,854 residential parcels in Cook County.
- 189,430 of them — about 1 in 8 — ended the year assessed below their certified value.
- $639 million in residential assessed value was removed through the appeal process.
- The same basic pattern holds every year back to 2019 — the total value removed moves with market conditions, but a consistent share of parcels sees a reduction every single cycle.
We report this as value removed through the Board of Review process, which is actually a conservative slice of the full picture: it doesn't include reductions the Assessor's office granted earlier in the cycle, before a case ever reached the Board. If anything, the true share of parcels touched by the appeal process in some form is higher than 1 in 8.
What this data does — and doesn't — prove
We want to be precise here, because it's the part most write-ups skip.
What it shows: a real, verifiable outcome measure — how many parcels in one of the largest counties in the country ended the year lower than where the county started them, and how much assessed value that represents.
What it does not show: the success rate of people who file appeals. Our dataset counts every parcel that ended up lower, regardless of whether the change came from an owner-filed appeal, a township-wide adjustment, a data correction, or something else entirely. It's an outcome measure across the whole parcel population, not a filer win rate.
Anyone telling you "X% of appeals win" needs to show appeal-level data — filings in, decisions out — and to our knowledge, no jurisdiction publishes that at the parcel level in a way we could independently verify. Cook County's own data gets us close, but "close" and "the filer success rate" are not the same claim, and we're not going to blur that line for a punchier headline.
Does any other jurisdiction publish real appeal outcome data?
We looked. Most county assessor and appraisal district sites publish appeal deadlines, forms, and general guidance, but few publish parcel-level, machine-readable outcome data the way Cook County does through its open data portal.
Where we couldn't independently verify a specific figure from an official source, we're leaving it out rather than repeating a number from a blog or content site with no citation attached — that's the entire point of this post. A single verified dataset, honestly described, is worth more than a page full of borrowed percentages. If you know of another county or state that publishes real, sourced appeal outcome data at this level of detail, we'd genuinely like to add it here.
The related — but different — national number
What we can cite reliably is the national over-assessment picture, which answers a related but different question: not "do appeals work," but "how many properties would have a case if their owner looked."
The National Taxpayers Union Foundation estimates that between 30 and 60 percent of taxable property in the U.S. is over-assessed, and that middle- and lower-income homeowners are disproportionately affected. Even the low end of that range means close to a third of homes could plausibly have a legitimate case — which lines up with why a meaningful, consistent share of Cook County parcels sees a reduction every year.
What actually drives a successful appeal
Outcomes aren't random. Across the appeal process generally, three categories of evidence do almost all the work.
1. Factual record errors
Wrong square footage, an extra bathroom that doesn't exist, a basement counted as finished when it isn't, or a condition rating that hasn't been updated in years. These are the cleanest wins because they're objective — the county's record is simply wrong, and correcting it doesn't require arguing about value at all.
2. Comparable sales
Recent, nearby, similar-in-size-and-condition sales that support a lower value than the assessment implies. This is the core evidence for a market-value argument, and it's what separates "my taxes feel high" from a filing an assessor's office will take seriously.
3. Uniformity and equity arguments
In many jurisdictions, you can appeal on the grounds that your home is assessed at a higher percentage of its value than comparable nearby properties, even if your own market value estimate is defensible on its own. This is underused because it requires pulling assessment ratios for a set of comparable homes, which is more work than most owners do alone. For more on how assessed value, market value, and assessment ratios relate, see our breakdown of assessed value vs. market value.
Realistic expectations
Set expectations before you file, not after.
Most successful outcomes are partial reductions, not dramatic ones. A modest correction — assessed value trimmed by single-digit or low-double-digit percentage points — is the typical result, not a 50% cut. Our Cook County figures reflect this: $639 million spread across 189,430 parcels averages out to a few thousand dollars of assessed value per parcel, not a windfall.
A reduction in assessed value doesn't always translate 1:1 into a lower bill. Where assessment caps, homestead exemptions, or levy-based tax calculations are in play, the relationship between "your assessed value went down" and "your bill went down by the same percentage" is not linear. Check how your specific jurisdiction's tax rate and exemptions interact with assessed value before assuming the math is simple.
Filing usually costs time, not money. Most counties charge nothing to file a residential appeal, which makes the realistic downside mostly the hour or two it takes to assemble evidence, not a financial risk if the appeal doesn't succeed.
Why most homeowners never file
If somewhere between 30 and 60 percent of properties are plausibly over-assessed, why doesn't everyone appeal? The same National Taxpayers Union Foundation estimate notes that fewer than 5 percent of taxpayers actually contest their assessment in a given year.
The gap between those two numbers is the real story. Most homeowners never open their assessment notice with a critical eye, don't realize a deadline is running, assume the county's number must be right because it's official, or find the process opaque enough that it's easier to just pay the bill. None of those are good reasons to skip a check that costs nothing — they're just what happens by default when nobody tells you otherwise.
Where to go from here
If you're not sure whether your own assessment holds up, our county-by-county guides cover deadlines, forms, and evidence requirements for the jurisdictions we've researched.
And if you just want a fast read on your own property: enter your address into our free check, and we'll compare your assessment against recent comparable sales in your area, flag anything that looks like a record error, and give you an honest answer — including "no case," which is a real answer we give often. We don't file appeals for you and we're not a law firm; we just do the research so you know whether filing is worth your time before you spend it.