Last updated 2026-07-09
TL;DR
Tax assessment value is the taxable value your assessor assigns to your property, usually a percentage of estimated market value. Multiply it by your local tax rate (the mill rate) and you get your yearly property tax bill. Assessments are wrong more often than people think, and most homeowners who appeal win at least a partial reduction, usually without hiring anyone.
What is tax assessment value, exactly?
Tax assessment value is the number your local government uses to start the math on your property tax bill. It is not what your house would sell for today, and it is not an appraisal. It is a bureaucratic estimate produced by your county or municipal assessor, usually on a fixed schedule, using mass-appraisal methods that run the same formulas across thousands of properties at once. [1]
The formula is simple. Assessed value times the local tax rate (expressed as mills, or dollars per $1,000 of value) equals your annual bill. If your assessed value is $300,000 and your combined mill rate is 20 mills ($20 per $1,000), your bill is $6,000 a year.
The assessed value on your notice may look like market value, or it may look oddly low. That depends entirely on your state's assessment ratio, which the next section covers. Assessed value and market value are related, but they are almost never the same number. Confusing the two is the single most common reason homeowners abandon appeals they could have won.
How does assessed value differ from market value and appraised value?
These three terms sound interchangeable. They are not.
Market value (also called fair market value) is the price a willing buyer would pay a willing seller in an arm's-length deal, with neither under pressure. It is what a licensed appraiser tries to estimate. It is also the standard most state statutes point to when they write assessment law, even when the actual assessed figure comes out lower. [2]
Appraised value is the conclusion a licensed appraiser reaches after a hands-on inspection and a formal report. You pay for a private appraisal. Your county assessor almost never orders one for each parcel. They run mass-appraisal models instead.
Assessed value is what the assessor produces after applying its models and, critically, its jurisdiction's legal assessment ratio. Some states assess at 100% of estimated market value. Many assess at a fraction, say 40%, 50%, or 80%. [3]
Here is a concrete example. Say the assessor estimates your home's market value at $400,000, and your state's ratio is 40%. Your assessed value is $160,000. That number lands on your tax notice, and the mill rate applies to $160,000, not $400,000. This is why a neighbor's "assessed value" can sound impossibly low until you understand the ratio behind it.
The table below shows how far assessed value can sit from estimated market value across a sample of states, based on published assessment ratios. [3]
| State | Typical assessment ratio | Example: $400K market value → assessed value |
|---|---|---|
| California (post-Prop 13) | ~100% of purchase price (capped) | Depends on purchase year |
| Illinois | 33.3% (residential) | ~$133,000 |
| Georgia | 40% | $160,000 |
| New York (varies by county) | 100% in some, <10% in others | Varies widely |
| Texas | 100% (no ratio; exemptions lower it) | $400,000 before exemptions |
| Colorado | 6.765% (residential, 2023 to 2024) | ~$27,000 |
Sources: state statutes and each state's department of revenue. Colorado's figure comes from a 2023 legislative change, Senate Bill 23-238. [4]
How do assessors actually calculate assessed value?
Most counties use mass appraisal, governed by standards from the International Association of Assessing Officers (IAAO). Mass appraisal means the assessor builds statistical models from recent sales, property characteristics, and neighborhood factors, then runs every parcel through the model. Nobody physically inspects your house every year. [1]
The three methods assessors use mirror what a private appraiser does.
The sales comparison approach looks at recent arm's-length sales of similar properties nearby, then adjusts for differences in size, age, condition, and amenities. This is the dominant method for houses.
The cost approach estimates what it would cost to rebuild the structure today, then subtracts depreciation. It shows up for newer construction or unusual properties where sales comps are thin.
The income approach capitalizes the net rental income a property could throw off. You see this almost only on commercial and income-producing property, not single-family homes.
A mass-appraisal model is only as good as its data. If your square footage is wrong in the assessor's records, or your house is coded with a finished basement it does not have, the output is wrong before any formula runs. Checking your property's data card, which most assessors now post online, is the first thing to do after a notice lands in your mailbox. [5]
For a closer look at how your local assessor runs these models, property assessment value breaks down the methodology by state.
What is the assessment ratio, and why does it matter so much?
The assessment ratio is the percentage of market value at which your jurisdiction is legally required to assess property. It decides whether the number on your tax notice is directly comparable to what your house is worth, or whether it is a fraction of that figure.
When a notice says $200,000 and your house is clearly worth $500,000, do not assume the assessor made a huge mistake. Check the statutory ratio first. In Illinois, the residential assessment ratio is 33.3%, so a $500,000 home should show an assessed value around $166,000. [3]
Here is why ratio awareness changes how you appeal. You win by showing the assessor's estimate of market value is too high, or by showing that similar properties were assessed at lower ratios (an equity argument). Walk into a hearing and say "my assessed value is $200,000 but my house is worth $500,000," and a hearing officer who knows the ratio is 40% will point out that $200,000 is exactly right for a $500,000 house. Argue that the underlying market value estimate is wrong. Do not argue the ratio math.
States change assessment ratios through legislation. Colorado's residential ratio dropped sharply in 2023 under Senate Bill 23-238 to ease the tax hit during a stretch of fast appreciation. [4] A change like that can shift your tax burden even when your home's market value never moves.
What is a mill rate, and how does it turn assessed value into a tax bill?
A mill rate (or millage rate) is the tax rate applied to your assessed value. One mill equals $1 of tax per $1,000 of assessed value. Your total mill rate stacks the levies from every taxing body that can bill your property: the county, the municipality, the school district, and any special districts for fire, water, or libraries. [6]
The math:
Assessed value ÷ 1,000 × mill rate = annual tax bill
Example: $250,000 ÷ 1,000 × 22 mills = $5,500 a year.
Mill rates are not fixed. Each taxing body recalculates its rate every year by dividing its budget by the total assessed value of all taxable property in its jurisdiction. If your assessed value climbs 15% but the school district's budget stays flat, the mill rate should drop by roughly the same amount and leave your bill unchanged. In practice, budgets go up, and the mill rate rarely falls far enough to cancel out the appreciation.
Your bill might list these separately ("school levy: 14.2 mills; county levy: 5.8 mills; library: 0.5 mills") or fold them into one line. The breakdown matters when you are trying to explain a jump. Was it your assessed value, or was it a mill rate hike passed by a taxing body?
For a wider view of how the whole system fits together, Property tax explained: how it's set and how to appeal it is a good next read.
Can your tax assessment value be wrong?
Yes. Often. IAAO performance standards say a well-run mass-appraisal system should produce a median assessment-to-sale-price ratio between 90% and 110%, with a coefficient of dispersion (a measure of uniformity) below 15% for residential property. [1] Read that again. Even a system running at the top of its profession allows assessments to be off by up to 15% in either direction before anyone flags a problem.
Plenty of jurisdictions never hit those standards. A 2021 study from the University of Chicago's Harris School of Public Policy found that low-value homes in Chicago were systematically over-assessed compared to high-value homes, a pattern the study found repeated across many U.S. counties. [7] The term for this is assessment regressivity: owners of cheaper properties pay higher effective tax rates than owners of expensive ones, even when the same ratio applies to everyone on paper.
Common reasons your assessment is wrong:
Your property's characteristics are recorded incorrectly (wrong square footage, bedroom count, lot size, or condition grade). The assessor leaned on sales that are not truly comparable. Your neighborhood saw price declines the model has not caught up to. New construction nearby is dragging up the model's estimates. You qualify for an exemption the assessor never applied.
Checking your property record costs nothing and takes minutes. It should be the first move any homeowner makes. Use your local property tax lookup or property tax records tool to pull your data card, then compare it to what you actually own.
How do property tax exemptions change your assessed value?
Exemptions cut your taxable assessed value, which cuts your bill. They do not touch the assessor's estimate of your home's market value. They carve a slice off the assessed value before the mill rate ever hits it.
The homestead exemption is the most common one. In Texas, the homestead exemption removes $100,000 from the assessed value of a primary residence for school district taxes, effective 2023, after voters approved a constitutional amendment in November 2023. [8] On a $300,000 home facing a 1.2% school levy, that exemption saves $1,200 a year on the school portion alone.
Other big exemption categories:
Senior and elderly exemptions: many states freeze assessed value or add a deduction for homeowners over a certain age. Income limits often apply.
Veteran and disability exemptions: these run from a flat dollar reduction to a full exemption for qualifying disabled veterans.
Agricultural exemptions: farmland is often assessed at its income-producing value instead of its market value for development.
Religious, nonprofit, and government exemptions: entire classes of property pulled off the tax rolls.
Exemptions are not automatic in most states. You apply, usually once, through your assessor's office or a state agency. Blowing the deadline happens all the time, and most states will not backdate. Check your assessor's website for the exact form and deadline in your county.
How often does your assessed value change?
This varies enormously by state and sometimes by county inside one state. There is no federal standard.
California is the outlier. Proposition 13 (1978) limits reassessment to when a property changes hands or gets new construction, so most owners never see a market-driven jump. [9]
Most other states reassess on a cycle. Many jurisdictions reassess every one, two, three, or four years. Michigan reassesses annually but caps the taxable value increase at 5% or inflation, whichever is lower, unless the property sells. [10]
In annual-reassessment states, your assessed value can move meaningfully every year. On longer cycles, a reassessment year can hand you a sticker-shock notice that reflects several years of appreciation all at once.
Prop 13 created a situation where neighbors in identical houses pay wildly different tax bills based only on when each one bought. The long-term owner who bought in 1992 might pay $1,800 a year while the buyer next door from 2022 pays $12,000. That gap is a feature of Prop 13's design, not a bug, and it is worth understanding if you live in California. [9]
In states that reassess often, appeal windows open on a fixed schedule tied to your assessment notice. Miss the window and you wait for the next cycle. For county-specific cycles and deadlines, local pages like clark county property tax and montgomery county property tax document the rules on the ground.
What is the difference between assessed value and taxable value?
Sometimes these terms mean the same thing. Sometimes they mean two different numbers. In states where they differ, the difference is money in your pocket.
Take Michigan. Assessed value (AV) is supposed to be 50% of market value, the constitutional standard. Taxable value (TV) is a separate number that starts at AV when you buy, then gets capped at the lesser of 5% growth or inflation each year you own the home. When the house sells, taxable value "uncaps" and resets to match assessed value. [10] Your bill runs off taxable value, not assessed value. Two identical houses with identical assessed values can carry very different taxable values and very different bills.
California works on a similar idea under Proposition 13: the assessed value for tax purposes is the purchase price plus no more than 2% a year, no matter what the market does. [9]
Texas uses "appraised value" for the assessor's market value estimate, and the comptroller's property tax division separates that from the "assessed value," which is the appraised value minus any exemptions or limitations. [8]
The takeaway: when you read your notice, find the exact line the bill is calculated from. That line is your real taxable base, whatever your jurisdiction calls it. Everything else is a step in the math, not the number the mill rate hits.
How do you appeal if your assessed value is too high?
The appeal process has a standard shape almost everywhere, even though the office names and deadlines change from place to place.
Step 1: Check your property record. Pull your data card from the assessor's website and confirm the square footage, bedroom and bathroom count, lot size, year built, and condition grade. Errors here are the easiest wins.
Step 2: Gather comparable sales. Find three to five recent sales of homes like yours, within the last six to twelve months, in your immediate neighborhood. Closer and more similar is better. Your assessor's office may post these. Zillow, Redfin, and your county recorder are also sources. The point is to show the assessor's market value estimate sits above what similar homes actually sell for.
Step 3: File by the deadline. Deadlines are hard. Most jurisdictions give you 30 to 90 days from the date on the assessment notice. Miss it and you forfeit the right to appeal that year. [5]
Step 4: Attend the informal hearing. Most systems run an informal review first, where you sit down with an assessor's staffer, present your evidence, and often settle it without going further.
Step 5: Escalate if you need to. If the informal review does not get you a fair result, you usually have the right to a formal hearing before an assessment review board or its equivalent. Beyond that, most states allow an appeal to tax court.
You do not need a property tax attorney or a contingency firm to do any of this. Contingency firms typically take 25 to 50% of your first year's savings. For most homeowners, the evidence and the hearing are well within reach with a structured approach. The TaxFightBack DIY appeal kit walks through the whole thing step by step so you keep 100% of what you save.
For county-specific details on what local assessors accept as evidence, see dekalb county tax assessor, bexar county property taxes, and denton county property tax.
What are realistic odds your appeal will succeed?
Nobody has great national data on this. The closest published figures come from state-level reports and the Lincoln Institute of Land Policy, which tracks property tax administration. What exists suggests homeowners who file formal appeals win some reduction in most cases where they bring comparable sales evidence, but success rates swing widely by jurisdiction, assessor culture, and the quality of the evidence. [11]
Here is what the data does show consistently: most homeowners never appeal. In many jurisdictions, fewer than 5% of property owners file any challenge in a given year. Of those who do, a large share get at least a partial reduction. The barrier is not a process that is too hard. It is that most owners do not know the process exists or assume it takes a lawyer.
Commercial property is different. Appeals are more common and the dollars are bigger, which is why attorneys and contingency consultants cluster there. For a house, a self-represented owner with solid comps and an accurate property record can do just as well.
The real question is not "will I win?" It is "is my assessed value more than 5 to 10% above what my house would realistically sell for?" If the answer is yes, one to two hours on an appeal is almost always worth it.
How does assessment value differ across major counties?
Local rules shape everything. The same idea of "assessed value" behaves one way in Orange County, California, another in Philadelphia, and another in Las Vegas.
In Orange County, California, assessed value is anchored to the purchase price under Prop 13, with a 2% annual cap. A homeowner who bought in 2005 and a buyer from 2022 in identical houses carry completely different assessments. The OC Assessor reassesses to market value only at sale. [9] See oc property tax for the local details.
In Philadelphia, the Office of Property Assessment runs citywide reassessments that have historically been irregular and have drawn legal challenges over uniformity. The city targets 100% of market value but has taken criticism for uneven results across neighborhoods. [7] More at philadelphia property tax.
In Clark County, Nevada (Las Vegas), the assessor sets assessed value at 35% of taxable value, and that taxable value reflects a depreciation-adjusted replacement cost method rather than a pure market approach. Nevada's cap on annual taxable value increases (3% for primary residences, up to 8% for other property) holds down year-over-year bill increases no matter what the market does. clark county property tax covers the mechanics.
In Loudoun County, Virginia, the assessor aims for 100% of market value with annual reassessments. Loudoun has been one of the fastest-appreciating markets in the country, which makes appeal evidence matter a lot for recent buyers. Details at loudoun county property tax.
The pattern is clear. States and counties make very different choices about ratio, frequency, and caps. Knowing your local rules is not optional if you want to judge whether your assessment is fair.
Frequently asked questions
Is tax assessment value the same as what my house is worth?
Rarely. Most states assess property at a fraction of estimated market value, called the assessment ratio. Even in states targeting 100% of market value, mass-appraisal models produce estimates that can run 10 to 20% above or below actual sale prices. Your tax assessment value is the number used to calculate your bill, not a professional opinion of what your home would sell for.
Does a higher assessed value always mean a higher tax bill?
Usually, but not always. Mill rates reset each year. If your assessed value rises but the taxing authority's budget stays flat, the mill rate falls and partly offsets the increase. In states with assessment caps like Michigan and California, your taxable value can stay flat even as assessed value rises. Check both your assessed value and your mill rate when you try to explain a bill change.
How do I find my property's assessed value?
Your county assessor's website almost always has a public search tool. Enter your address and look for the current assessment, tax year, and property data card. If you cannot find it online, call or visit the assessor's office. Your annual tax notice also states it. The assessed value is the figure before exemptions. The taxable value after exemptions is the one the mill rate actually hits.
What does it mean when assessed value is lower than market value?
It means your state uses a fractional assessment ratio below 100%. If your state assesses at 40% and your home is worth $500,000, your assessed value should be around $200,000. This is legal and intentional; the mill rate is calibrated to the ratio. The question for homeowners is not whether assessed value equals market value, but whether the ratio is applied consistently across similar properties.
Can my assessment go up even if I did not sell or renovate my home?
Yes, in most states. Annual or cyclic reassessments adjust values to reflect the local market, even if your home sits untouched. California under Prop 13 is the major exception: assessed value is capped at purchase price plus 2% a year until sale or new construction. Most other states allow market-driven increases on a regular schedule.
What is a notice of assessed value, and when do I get it?
A notice of assessed value (sometimes called a reassessment notice or change of assessment notice) is the official document your assessor mails when your assessed value changes. It usually arrives before your tax bill. It starts the clock on your appeal deadline, commonly 30 to 90 days from the notice date. Read it right away, because missing that window means waiting another full cycle.
Does getting an appraisal help lower my assessed value?
A private appraisal from a licensed appraiser is strong evidence in an appeal because it addresses the assessor's market value estimate with a credentialed opinion. It costs $300 to $600 for most homes. For modest potential savings, it can cost more than the reduction is worth. For larger over-assessments, the math usually favors getting one. Comparable sales pulled from your county's own records can be just as persuasive and cost nothing.
What is the difference between assessed value and taxable value in Michigan?
Michigan's constitution sets assessed value at 50% of market value. Taxable value starts at that level when you buy and is then capped at the lesser of 5% or inflation each year you own. When the property sells, taxable value resets to match assessed value. Your bill runs off taxable value, so long-term owners often pay taxes on a number well below 50% of current market value.
How do property tax exemptions affect assessed value?
Exemptions cut your taxable assessed value, which cuts your bill. They do not change what the assessor thinks your home is worth. Common exemptions include homestead, senior, veteran, and disability programs. Most require an application and have deadlines. Texas raised its homestead exemption to $100,000 for school district taxes in 2023. Missing an exemption you qualify for is one of the most common and most fixable property tax mistakes.
Can I appeal my property tax assessment without a lawyer?
Yes. Most residential appeals start with an informal review, where you present comparable sales and your property data card to an assessor's representative. No legal training required. Formal hearings before assessment review boards are also built for self-represented homeowners. Attorneys and contingency consultants cluster in commercial cases where the stakes justify the cost. For most homeowners, the tools are comps and accurate property records.
What is an equity appeal in property taxes?
An equity appeal argues that your property was assessed at a higher ratio than comparable properties, even if the dollar value looks defensible. If similar homes in your neighborhood sit at 85% of market value but yours is at 105%, you have an equity argument under the uniformity clause most state constitutions carry. You do not have to prove the market value estimate is wrong, only that you were treated worse than your neighbors.
How much can I realistically save by appealing my property tax assessment?
It depends on how far off your assessment is and your local mill rate. A $50,000 reduction in assessed value in a 20-mill jurisdiction saves $1,000 a year. A $100,000 reduction saves $2,000. Savings compound over the years you own because a lower assessed value is the base for future increases. Nobody has good national averages; state-level studies typically report a few hundred to a few thousand dollars per successful residential appeal.
What should I do first after getting a high assessment notice?
Pull your property data card from the assessor's website and check every recorded characteristic against your actual house. Wrong square footage, extra bathrooms that do not exist, or a finished basement that is really unfinished are the easiest wins because they are factual errors, not valuation arguments. Then look up recent sales of comparable homes nearby. Those two steps, done in an hour or two, tell you whether you have a real case before you file anything.
Sources
- International Association of Assessing Officers (IAAO), Standard on Mass Appraisal of Real Property: Mass appraisal methodology standards and acceptable coefficient of dispersion ranges for residential property assessment
- IRS, Fair Market Value definition (Publication 561): Fair market value defined as the price a willing buyer would pay a willing seller with neither under compulsion
- Lincoln Institute of Land Policy, Significant Features of the Property Tax (State assessment ratio data): Assessment ratios by state, including Illinois at 33.3% residential and Georgia at 40%
- Colorado General Assembly, Senate Bill 23-238 (Reduce Property Tax Revenue): Colorado's residential assessment ratio reduced to 6.765% for 2023-2024 tax years under SB 23-238
- National Taxpayers Union Foundation, Property Tax Appeal Guide: Most jurisdictions allow 30 to 90 days from assessment notice date to file a property tax appeal
- Tax Foundation, Property Taxes: A Primer: Mill rate definition: one mill equals $1 of tax per $1,000 of assessed value; total rate combines multiple levies
- University of Chicago Harris School of Public Policy, The Regressivity of the Property Tax (2021): Study found low-value homes were systematically over-assessed relative to high-value homes in Chicago and many other U.S. counties
- Texas Comptroller of Public Accounts, Property Tax Exemptions: Texas homestead exemption increased to $100,000 for school district taxes effective 2023 after voter approval
- California State Board of Equalization, Proposition 13 Overview: California Prop 13 caps assessed value at purchase price plus maximum 2% annual increase until sale or new construction
- Michigan Department of Treasury, Assessed Value vs. Taxable Value: Michigan assesses at 50% of market value; taxable value is capped annually at 5% or inflation, whichever is lower, and uncaps at sale
- Lincoln Institute of Land Policy, Property Tax Assessment Administration: Fewer than 5% of property owners file appeals in most jurisdictions; a significant share of those who do receive at least a partial reduction