What Is Overassessment?
When a property's assessed value exceeds its actual market value, resulting in the homeowner paying more property tax than they should.
Detailed Explanation
Overassessment occurs when your county's estimated value of your property is higher than what it would actually sell for on the open market. It is far more common than most homeowners realize. Mass appraisal values every property with a model, so a meaningful share of residential properties carry an assessment above what they would actually sell for in any given year, and the share varies with the market and the county. Overassessment happens for several reasons. Mass appraisal models cannot account for every property's unique characteristics. Data errors in county records (wrong square footage, extra bedrooms, inaccurate lot size) inflate values. Rapidly changing markets leave assessments outdated, especially in declining neighborhoods where the county has not caught up. Physical issues like foundation problems, flood damage, or proximity to commercial nuisances are rarely captured. The consequences compound over time. An overassessment of $30,000 on a property in a state with a 2% effective tax rate costs $600 per year. Over five years, that is $3,000 in taxes you did not owe. Unlike a billing error you might catch immediately, overassessment quietly drains your finances year after year until you challenge it. The only way to correct an overassessment is to file an appeal. Counties do not proactively lower values for individual homeowners. Overassessment is also not distributed evenly. Economist Christopher Berry's research at the University of Chicago's Center for Municipal Finance, using millions of residential sales, found that assessments are typically regressive: within the same jurisdiction, homes in the bottom 10% of sale price are assessed, as a share of what they actually sold for, at roughly twice the rate of homes in the top 10%. In other words, lower-value homes are systematically more likely to be overassessed relative to their worth than higher-value homes are, which is the opposite of what most people assume about who gets hit hardest by assessment errors.
How It Varies by State
Annual reassessment means CADs often raise values aggressively in hot markets. The 10% homestead cap can create distortions when market softens.
Properties in reassessment years may see large value jumps that overshoot market reality. Cook County sees one of the highest appeal volumes of any county in the country. Cook County is also the jurisdiction Christopher Berry's regressivity research is most closely associated with.
With the highest effective property tax rates in the nation (avg 2.23%), even small overassessments are expensive.
A $50,000 overestimate of market value translates to only $20,000 in assessed value, but at 1.0% effective rate still costs $200/year.
Common Misconceptions
Myth:If the county raises my assessment, they must be right
Reality:Counties use statistical models, not individual inspections. These models often lag behind market conditions or contain data errors. You have the legal right to challenge the result.
Myth:Only expensive homes get overassessed
Reality:University of Chicago research (Christopher Berry, Center for Municipal Finance) found the opposite pattern: homes in the bottom decile of sale price are assessed at roughly double the rate, as a share of price, of homes in the top decile. Overassessment hits lower-value homes disproportionately hard, not high-value ones.
Myth:Filing an appeal might cause the county to raise my value even higher
Reality:In most states, an appeal can only result in the assessment staying the same or going down. A few states technically allow increases, but this is extremely rare in practice.
Impact on Your Tax Bill
In New Jersey, if your home is worth $425,000 but the county has it assessed at $475,000 (a $50,000 overassessment), you are overpaying. At NJ's average effective rate of 2.23%, that $50,000 error costs you $1,115 per year. Over 5 years without an appeal, that is $5,575 in unnecessary taxes. Berry's research suggests this burden falls hardest, proportionally, on lower-value homes in a given jurisdiction, so a modest home carrying a $20,000 overassessment can be losing a larger share of its actual value to excess tax than a much more expensive home with a similar dollar-value error.
Frequently Asked Questions
Are cheaper homes really more likely to be overassessed than expensive ones?
How common is overassessment, really?
Will the county automatically fix an overassessment if I point it out informally?
Does overassessment ever correct itself over time?
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