What Is a Property Tax Assessment? How Assessors Value Your Home (and Why It Doesn't Match Market Value)
A property tax assessment is the county or municipal assessor's estimate of what your home is worth for tax purposes — the number that, once multiplied by an assessment ratio and a millage rate, becomes your annual property tax bill. It is not the same as your home's market value, it is not an appraisal, and in most jurisdictions it lags what your home could actually sell for today by anywhere from a few percent to more than 40%. This guide walks through what the number means, how assessors arrive at it, how often it gets updated in your state, how the millage math works, when it's worth appealing, and which exemptions can lower it. If you want the broader context of how home value is calculated across every valuation method, the assessment is only one piece — and often the least accurate one.
Key takeaways
- A property tax assessment is a government estimate of your home's value used to calculate property tax — not market value and not a mortgage appraisal (Investopedia).
- Assessors use mass appraisal — statistical models applied to thousands of homes at once using recent comparable sales, square footage, lot size, and building characteristics (IAAO Standard on Mass Appraisal).
- Reassessment cycles vary sharply by state: California caps growth at 2%/yr until sale (Prop 13), Florida caps homestead growth at 3% (Save Our Homes), Texas reassesses annually, Ohio does a full reappraisal every six years (Lincoln Institute — 50-state property tax comparison).
- Your annual bill roughly equals (market value × assessment ratio) × millage ÷ 1,000. A $400,000 home at 80% ratio and 15 mills is a $4,800 bill.
- 30–60% of appealed residential assessments result in some reduction, but only 2–5% of homeowners appeal (NTU Foundation).
What is a property tax assessment?
A property tax assessment is the value your local assessor assigns to your home for the purpose of calculating property tax. It's set by the county or municipal assessor on a schedule your state controls, and it's what gets multiplied by the local tax rate to produce your bill. Assessment is a civic process, not a market one — the goal isn't to price your house for sale but to distribute the local property tax burden proportionally across every taxable parcel in the jurisdiction.
The number lands as an assessment notice once a year, or once every few years depending on your state's cycle. That notice does not ask you for money. The tax bill — a separate document from a separate office, usually the treasurer or tax collector — arrives later and uses the assessed value as one of its inputs.
Assessment vs tax bill — two different numbers
The assessment is the value; the tax bill is the value multiplied by the tax rate. Changing the assessment changes the bill, but they are separate numbers from often separate offices. If you disagree with the value, you appeal the assessment. If you disagree with the amount owed, that's a tax-collection question.
The distinction matters because homeowners frequently mix the two up. If your bill jumped this year, ask which number moved: the assessed value, the millage rate (the local government voted to raise taxes), or an exemption you lost. Only the first is appealable at the assessor's office.
Assessed value vs market value (why they rarely match)
Market value is what a willing buyer would pay a willing seller today; assessed value is what the assessor calculated as of a statutory valuation date, using mass-appraisal models on data that may be a year or several years old. The two numbers rarely match, and in most states they aren't supposed to. If you want the full breakdown, see fair market value and how market value vs appraised value both differ from assessed value.
Three things drive the gap. First, most states apply an assessment ratio — a deliberate discount. In a jurisdiction with an 80% ratio, a $400,000 market-value home is assessed at $320,000. Second, cycle lag — if your state reassesses every three or six years, your current assessed value reflects the market as of the last cycle, not today. Third, statutory caps — California's Prop 13 caps growth at 2% per year until the property changes hands; Florida's Save Our Homes caps homestead growth at 3%. In fast-appreciating markets, a long-tenured homeowner can have an assessed value 30–50% below current market value.
How assessors actually value your home (mass appraisal + comps)
Assessors don't visit every property every year. They run mass appraisal — statistical regression models that weigh recent comparable sales, square footage, lot size, year built, quality grade, condition, and neighborhood factors against your property record card (IAAO Standard on Mass Appraisal). The model produces a value for tens of thousands of parcels at once; the assessor spot-checks outliers.
Field inspections happen periodically (typically every 4–8 years) or when a permit is pulled. If the record card is wrong — wrong square footage, wrong bedroom count, missing damage — your assessment will be wrong too, because the model treats the card as ground truth. That's the first thing to check when you think an assessment is high.
Assessors also split the parcel into a land value and an improvement value (the house and structures). Both sum to the total on your notice. In markets where land has appreciated faster than the structure, the land line usually drives the year-over-year increase.
Assessment cycles by state — annual, biennial, or long-cycle
Reassessment frequency and assessment ratio vary sharply by state, which is why identical homes in different states can carry very different tax bills. Check your state Department of Revenue for exact rules where you live.
| State | Reassessment frequency | Assessment ratio | Notable cap or rule |
|---|---|---|---|
| California | On sale, then annual | 100% of base year value | Prop 13: 2%/yr growth cap until property changes hands |
| Texas | Annual | 100% of market value | 10% annual cap on homestead taxable value increases |
| Florida | Annual | 100% of just value | Save Our Homes: 3%/yr cap on homestead assessed value |
| New York | Annual (varies by municipality) | Varies widely | State equalization rate normalizes across jurisdictions |
| Illinois | Triennial in Cook County | 10% (Cook Co. residential) | Reassessed 1/3 of townships per year |
| Ohio | Triennial update + six-year full reappraisal | 35% of true value | HB 920 rollback limits levy growth |
| Michigan | Annual | 50% of true cash value | Proposal A: taxable value growth capped at CPI or 5% |
| Georgia | Annual | 40% of fair market value | Homestead freeze available in some counties |
| North Carolina | Up to 8-year cycle (county option) | 100% of appraised value | Most counties reassess every 4 years |
| Pennsylvania | Varies widely by county (some >30 yr) | Varies (common-level ratio) | Some counties have not reassessed since the 1980s |
The cycle explains why a home that has doubled in market value over five years may still show an assessed value near what it was worth three or four years ago — and why long-tenured California and Florida homeowners often pay dramatically less property tax than a neighbor who bought last year.
The math — assessment ratio × millage = your bill
The formula is: (market value × assessment ratio) × millage rate ÷ 1,000 = annual property tax. Walk it with a real number. A $400,000 home in a jurisdiction with an 80% assessment ratio has an assessed value of $320,000. Apply 15 mills — 15 dollars per $1,000 of assessed value — and the annual tax is $320,000 × 0.015 = $4,800.
A "mill" is one dollar per $1,000 of assessed value. Some jurisdictions publish the rate as a percentage (1.5%) or as dollars per $100 of value ($1.50 per $100) — same math, different label. If your state uses an effective tax rate expressed as a percentage of market value, that already folds the assessment ratio into the rate: a 1.2% effective rate on a $400,000 home is $4,800.
Two lines can shift the result. Exemptions subtract from assessed value before the rate is applied — a $50,000 homestead exemption on a $320,000 assessed value drops the bill to $4,050. Millage layers add up: your total rate is usually a sum of county, city, school district, and special-district levies, each shown separately.
How to read your assessment notice, line by line
Notices vary by state, but almost all include the same core fields.
- Parcel ID or account number — the unique identifier for your property. Use this on any appeal filing.
- Prior and current year assessed value — the year-over-year change is the first thing to scan. A 15% jump when nothing has changed about your house is worth investigating.
- Land value and improvement value — the two components that sum to total assessed value. If the improvement value jumped, ask whether a permit-triggered inspection updated the record card.
- Applied exemptions — homestead, senior, veteran, disability, agricultural. Confirm each one you're entitled to is showing.
- Taxable assessed value — assessed value minus exemptions. This is what the millage rate is applied to.
- Effective date of valuation — the statutory date the assessor is valuing as of, often January 1 or July 1 of the prior year.
- Appeal deadline — usually 30 to 60 days from the notice date, and in most jurisdictions there are no exceptions after. Miss it and you wait a full cycle.
Most notices also include contact information for informal review — a call or online form that lets you flag record-card errors before filing a formal appeal. Informal review resolves a meaningful share of homeowner concerns without a hearing.
Exemptions that reduce your assessment
Exemptions subtract from assessed value before the tax rate applies. Most are opt-in — you have to apply, and in most states they do not apply retroactively.
| Exemption | Typical eligibility | Typical range |
|---|---|---|
| Homestead | Primary residence, owner-occupied | $5,000–$50,000+ off assessed value; some states cap growth instead |
| Senior (65+) | Age 65+, often income-tested | Additional $10,000–$50,000 off; some states freeze value |
| Veteran / disabled veteran | Service-connected; disability rating scales the amount | Partial to 100% depending on rating |
| Widow / widower | Surviving spouse of qualifying decedent | $500–$5,000 off |
| Disability | Total and permanent disability | Full or partial exemption |
| Agricultural use | Land actively used for farming | Valued at agricultural rather than market value |
| Historic | Designated historic property | Rate freeze or reduced assessment for a term of years |
Reapply after a move, a change in marital status, a change in disability status, or a change in age eligibility. Homestead is the one homeowners lose most often — usually because they moved and didn't refile at the new address. The National Taxpayers Union Foundation (NTU Foundation) publishes overviews of exemption categories; for exact dollar figures and eligibility rules, check your state Department of Revenue or county assessor.
When (and how) to appeal your assessment
Appeal when the record card has factual errors, when recent comparable sales support a lower value than the assessor's, or when your assessed value is out of line with similar nearby homes. 30–60% of appealed residential assessments result in some reduction, but only 2–5% of homeowners appeal (NTUF — how to challenge your assessment). Most homeowners leave money on the table because they never file.
The process, generalized across most jurisdictions:
- Request the property record card and verify square footage, bedroom and bathroom count, lot size, year built, and condition rating. Errors here are the strongest appeal basis.
- Pull 3–5 recent comparable sales — homes within roughly half a mile, similar in size and condition, sold in the last 6–12 months. Assessor websites often publish comparable-sales tools.
- File the appeal by the deadline with your evidence. Most jurisdictions accept an online form. Attach the record-card corrections and comps.
- Attend the informal hearing with the assessor's staff. A significant share of appeals resolve here.
- Escalate to the county board of equalization if the informal review doesn't produce a satisfactory result.
When appeal is the wrong move: if the record card is accurate, recent comparable sales support the assessor's value, and your assessment is in line with similar nearby homes, an appeal will not succeed. Look for a specific error or a specific comparable that undercuts the assessor's number — not a general feeling that the number is high.
Property tax rules vary by state and county — check your assessor's website for exact deadlines and consult a property tax attorney or CPA if the amount at stake is significant.
Does the assessment affect what a buyer will pay?
Not directly. Buyers care about what similar homes have actually sold for and the ongoing carrying cost of the property, not the assessed value line on the tax notice. A low assessment doesn't make your house cheaper to sell; a high assessment doesn't make it more expensive. What can matter is the resulting tax bill — a sudden reassessment on sale (common in California under Prop 13) can add hundreds of dollars per month to the buyer's carrying cost. If you want a number that tracks what your home would actually sell for, check your home's current market value using recent comparable sales, not the assessed value on your tax notice.
The number on your assessment notice is a government tax figure — it doesn't dictate what a buyer will pay, and it doesn't dictate what Opendoor will pay, either. Opendoor's cash offer is built from recent comparable sales transactions from the past few months and the actual condition of your home (Opendoor Help Center — how offer price is determined), not from any assessed value figure. If you want a real market-based number to compare against the assessed value on your notice, see how Opendoor calculates the value of your home.
Common mistakes homeowners make with the assessment notice
Four errors show up over and over. Ignoring the notice — the appeal window is 30–60 days and there are usually no exceptions after. Appealing the tax bill instead of the assessed value — the bill isn't appealable at the assessor's office; the assessed value is. If the millage rate moved, that's a budget question for county commissioners and school boards, not the assessor. Appealing without comparable sales — "my taxes went up too much" rarely succeeds; "three homes within half a mile of similar size sold last quarter below my assessed value" often does. Not reapplying for exemptions after a move — homestead doesn't transfer automatically, and forgetting to refile can cost a $5,000–$50,000 reduction.
Disclosure
This material is provided for informational purposes only. Property tax laws, assessment cycles, exemption amounts, and appeal procedures vary by state and county — consult your county assessor, your state Department of Revenue, or a licensed property tax attorney or CPA for guidance on your specific situation. Opendoor does not provide tax advice.
