If you have ever looked up your home on Zillow, Redfin, or your county's website and seen three different values — a Zestimate, an assessed value, and maybe an appraised value — you might reasonably wonder which one actually matters for your property taxes. The answer is assessed value. But understanding why assessed value often differs significantly from market value — and what to do when the difference works against you — is one of the most practically useful things a homeowner can know.
Market Value: What Buyers Would Pay Today
Market value (also called fair market value) is the price a knowledgeable, willing buyer would pay a knowledgeable, willing seller in an arm's-length transaction, with no unusual pressure on either party. It is essentially what your home would sell for if you listed it today under normal market conditions.
Automated valuation models like Zillow's Zestimate or Redfin's Estimate try to calculate market value using algorithms that analyze recent comparable sales, property characteristics, and market trends. These models are generally accurate within 5–10% in areas with frequent sales, but can be significantly off in rural areas, neighborhoods with limited sales data, or for unusual properties.
When a licensed appraiser values your home for a mortgage lender, they are estimating market value using a more rigorous methodology — in-person inspection, recent comparable sales analysis, and professional judgment. Their appraisal is a formal opinion of market value.
Assessed Value: What the Government Uses for Tax Purposes
Assessed value is the value your local county assessor assigns to your property for the purpose of calculating property taxes. It is set by government, not the market, and it does not have to equal market value.
In some states, the goal is full-value assessment: assessed value should equal 100% of market value. New York State law requires this in principle, though actual practice varies. Connecticut, Massachusetts, and New Jersey also aim for full-value assessment.
In many other states, the law requires assessment at a specified fraction of market value — called the assessment ratio or assessment level. Illinois requires 33.33% of market value (one-third). South Carolina assesses primary residences at 4% of market value. Colorado has historically assessed residential property at a low ratio set by state law.
Even in states that require full-value assessment, the actual assessed values often lag behind market values due to infrequent reassessment cycles. A property last assessed in 2020 at $300,000 that is now worth $450,000 may still carry the $300,000 assessed value until the next reassessment.
Why Assessment Ratios Exist
Assessment ratios exist for several historical and administrative reasons. When property tax systems were first formalized in the 19th and early 20th centuries, full-value assessment was often impractical — assessors lacked the tools and data to accurately estimate market value for every property. Assessing at a fraction of estimated value introduced a buffer for imprecision.
Assessment ratios also served political purposes. Publicly announced tax rates (mill rates) appear lower when applied to a fraction of market value. A 30-mill rate on 33% of market value produces the same effective burden as a 10-mill rate on full market value — but 30 mills sounds more tolerable than it might, and 10 mills might sound low enough to invite higher levies.
Today, some states have simplified to full-value assessment, but many retain fractional systems for reasons of administrative tradition, political inertia, or explicit policy goals (such as South Carolina's 4% assessment ratio for owner-occupied homes, which significantly reduces the property tax burden on primary residence owners compared to investment properties at 6%).
How Assessors Determine Assessed Value
County assessors do not individually inspect every property every year. Most use a process called mass appraisal — a statistical system that applies valuation models to all properties simultaneously based on their characteristics and recent sales data in their area.
The assessor's computer-assisted mass appraisal (CAMA) system contains a property record for each parcel in the county. The record includes characteristics: square footage, lot size, number of bedrooms and bathrooms, year built, construction quality, and condition rating. The system uses sales data from recent transactions to calibrate how much each characteristic contributes to value in a given neighborhood.
Assessment lag is a structural feature of mass appraisal: assessors typically use sales data from the prior year or even prior few years. In rapidly appreciating markets, this creates a built-in delay between market value increases and assessed value increases. In declining markets, assessed values may remain above actual market value for a period before reassessment catches up — which is one reason to file an appeal when markets soften.
Assessment Cycles and How Often Values Change
Tip
In states without sale-triggered reassessment, your assessed value may still be close to what the previous owner paid for the home years ago — especially if the assessor is on a multi-year cycle. This can work in your favor if the market has appreciated significantly.
Most states require reassessment at specified intervals. California reassesses only when a property is sold (or improved). Texas, Illinois, New York, and most other states reassess on a schedule ranging from annually to every four or five years. In jurisdictions with long reassessment cycles, assessed values can fall significantly behind market values in rapidly appreciating markets.
Some states trigger reassessment when a property is sold — California's Prop 13 is the most famous example, resetting assessed value to the purchase price whenever the property changes ownership. Massachusetts, Oregon, and Michigan have similar (if less dramatic) mechanisms that link assessed value to recent sale events.
When Your Assessed Value Is Too High — and What to Do
If your assessed value is above what your home would actually sell for in the current market, you are likely being overtaxed. This situation is more common than most people realize, particularly in the year or two after a market correction, when assessed values from a peak have not yet been adjusted downward.
To check: compare your assessed value (found on your county assessor's website) to the recent sale prices of similar homes in your neighborhood. If comparable homes have sold for less than what your assessed value implies they are worth — using your jurisdiction's assessment ratio — you have grounds for an appeal.
Request your property record card from the assessor's office and check every input: square footage, room count, lot size, year built, condition rating. Errors in these inputs are surprisingly common and can cause significant overvaluation. If the card shows your home has more bedrooms than it actually does, the overvaluation may be entirely due to a data error that can be corrected without a formal appeal.
File an appeal with your local Board of Review or Assessment Appeals Board before the deadline (which varies by jurisdiction but is typically in the spring or summer). Bring comparable sales data, your property record card, and any documentation of condition issues that reduce your home's value.
Why This Matters: The Real-World Impact
The gap between assessed value and market value has direct financial consequences. In states with high effective tax rates (1.5–3.0%), a $50,000 overassessment can cost you $750 to $1,500 per year in additional taxes. Over ten years, that is $7,500 to $15,000 — simply because of inaccurate government data that you have the legal right to challenge.
Conversely, understanding the assessment ratio in your state can help you evaluate whether your tax bill is reasonable. If you live in a state with a 33% assessment ratio and your home is worth $600,000, your assessed value should be approximately $200,000. If it is $280,000, you are being assessed at a higher ratio than the law requires — and that is grounds for an appeal.
PropertyTaxByZip shows effective tax rates derived from Census data (median taxes paid divided by median home value), which gives you a market-value-based benchmark for your ZIP code. If your personal effective rate is significantly above the ZIP code median, it is worth investigating whether your assessed value is accurate.
Data Source
ZIP-level property tax data on PropertyTaxByZip comes from the U.S. Census Bureau, American Community Survey 2019-2023 5-Year Estimates. Effective rates shown on this site are derived by dividing median real estate taxes paid by median home value — both as reported by survey respondents. This reflects a market-value-based effective rate, which may differ from the statutory assessment ratio in your state.
Data from U.S. Census Bureau, American Community Survey 2019-2023 5-Year Estimates (ZCTA level). All figures are estimates. This article is for informational purposes only and should not be considered financial, legal, or tax advice.