County appraisals are built to be roughly accurate across a neighborhood, not precisely accurate for your house. The industry standard set by the International Association of Assessing Officers treats an assessment as acceptable when it lands anywhere between 90 and 110 percent of a property’s actual market value.1IAAO. Standard on Ratio Studies – Exposure Draft September 2025 That means the county’s number can be ten percent above or below what a buyer would pay and still count as a job done well. For individual homes with unusual features, recent damage, or unpermitted upgrades, the miss is often larger.
Why the County’s Number Is Often Off
Assessors don’t walk through every house. They use mass appraisal, which values thousands of properties at once using statistical models rather than individual inspections. Most offices run Computer-Assisted Mass Appraisal software that feeds property characteristics — living area, year built, lot size, garage space, condition rating — into regression formulas calibrated on recent sales. The system groups similar properties together and applies the same pricing formula to the cluster. If comparable homes in your neighborhood sold around $340,000, the model assigns a similar figure to every house matching that profile.
It doesn’t know about your renovated kitchen, your neighbor’s leaky basement, or the retention pond behind your yard. A fee appraisal ordered by a mortgage lender involves a licensed appraiser walking the property, noting upgrades and deficiencies, and choosing comparable sales tailored to that specific house. Mass appraisal trades that precision for scale. The model is calibrated to be right on average across a neighborhood, not for any one house.
The Snapshot Is Stale by the Time You See It
Assessments reflect market conditions on a single date, usually January 1 of the tax year. Prices that moved after that date aren’t in the number. The gap widens in jurisdictions that only reassess every few years. State-mandated revaluation cycles range from annual updates to intervals as long as eight or ten years, with most states falling somewhere between one and five. In a hot market, a home revalued three years ago can carry an assessment $50,000 or more below its current sale price. In a declining market, the reverse happens, and owners get stuck paying taxes on yesterday’s peak.
The County Sees the Outside
Assessors generally work from building permits, aerial imagery, and original construction data. If you gutted your kitchen and added $40,000 in upgrades without pulling a permit, the county has no idea. If your basement floods every spring or the foundation has shifted, the model doesn’t reflect the loss unless you tell them. This information gap cuts both directions and is one of the most common reasons an individual assessment misses.
Non-Disclosure States Have Less to Work With
Roughly a dozen states, including Texas, Utah, Idaho, Montana, and Wyoming, don’t require sale prices to appear in public records. When assessors can’t see what properties actually sold for, they’re building models with less reliable data. Academic research has found that appraisal bias tends to be larger in these non-disclosure environments because less information is available to calibrate and check the models. If you live in one of these states, expect a wider spread between the county’s value and market reality.
A Low Tax Value Doesn’t Always Mean a Low Market Value
Before you decide the county is wrong, know that “tax value” and “market value” are measuring different things in many states by design.
Most states apply an assessment ratio that reduces the taxable figure to a fraction of appraised value. A state might assess residential property at 10 percent of market value, so a home appraised at $300,000 has an assessed value of $30,000. Other states assess at 100 percent. Homeowners sometimes panic at a high appraised value without realizing their state only taxes a slice of it.
Many states also cap how much assessed value can rise each year. California’s Proposition 13 restricts annual assessment growth to two percent, with reassessment to full market value only when the property changes hands or undergoes new construction. Florida’s Save Our Homes provision caps annual increases at three percent for homesteaded properties. Similar caps exist in Oregon, Iowa, Oklahoma, New York, and at least a dozen other states, with limits ranging from two to fifteen percent depending on the jurisdiction and property type. A long-term owner whose market value has doubled may still be assessed at a fraction of that. The cap resets when the property sells, which is why a new neighbor in a similar house can pay significantly more in property taxes than you do.
On top of that, most states subtract exemptions before applying the tax rate: homestead exemptions for owner-occupants, assessment freezes or enhanced exemptions for seniors (typically 62 to 65 with an income cap), and partial or full exemptions for disabled veterans tied to their VA rating. These usually require an application, and missing the deadline means paying the full amount until the next year.
So a valuation that looks low may just reflect a cap or exemption, not an actual accuracy problem. What matters for an appeal is whether the appraised value — the number before ratios, caps, and exemptions — is higher than what the property would fetch on the open market.
How to Check Your Assessment for Errors
Before filing anything, look at the county’s data. This is where the easiest wins are.
Read the Property Record Card
Every assessed property has a record card listing what the county used to calculate value: square footage, bedroom and bathroom counts, year built, lot size, garage type, and condition rating. These cards are usually on the assessor’s website. Errors are more common than you’d expect. A phantom half-bath, a finished basement coded when yours is unfinished, or a wrong year of construction can each add thousands to the value. Correcting a factual error is the most straightforward path to a reduction because you aren’t arguing opinion; you’re pointing out that the math started from wrong inputs.
Pull Comparable Sales
If the record card is accurate but the value still looks high, find three to five sales of similar homes in your area that closed within six months of the assessment date. Stick to arm’s-length transactions where both parties acted voluntarily at market price. Exclude foreclosures, estate sales, and family transfers. Match on square footage, age, lot size, and condition. If those sales consistently came in below your assessed value, you have an argument.
Document What Only Affects Your Property
The mass appraisal model doesn’t know about issues specific to your lot. Foundation damage, persistent water intrusion, environmental contamination on or near the property, proximity to a noisy highway, or a neighboring commercial use that depresses residential values all qualify. Assessors call the last group “external obsolescence,” and it’s a legitimate basis for reduction. Bring evidence: repair estimates, environmental reports, photos, anything that puts a dollar figure on the impact. A contractor’s letter saying foundation repair will cost $25,000 carries more weight than a verbal description of a cracked wall.
For higher-value properties, a licensed appraiser’s formal opinion of value can be strong evidence. A residential appraisal typically costs $300 to $500, so it only makes sense when the potential tax savings justify it.
Filing a Formal Appeal
Appeals follow a similar sequence almost everywhere, though the details vary by state.
Watch the Deadline
The window opens when assessment notices are mailed and typically closes 30 to 60 days later. Some states set fixed calendar dates, such as May 15 in Texas or April 1 in New Jersey. Miss it and you’re locked in for the year in nearly every jurisdiction. A few states allow late filings for good cause, but that’s not a plan. Mark the calendar the day the notice arrives.
Start With the Informal Review
Most assessor offices offer an informal review before any formal hearing. It’s a conversation. You present your evidence, the assessor explains their methodology, and in many cases the two sides reach a compromise. A surprising share of disputes end here, especially when the issue is a factual error on the record card. If nothing gets resolved, nothing you said can be used against you at the formal hearing.
The Formal Hearing
When informal talks fail, the case goes to a local review board — Board of Equalization, Assessment Appeals Board, or Appraisal Review Board depending on the state. You file a written protest form, usually available on the assessor’s website, with filing fees that generally range from nothing to under $100. You and the assessor each present evidence, and the board issues a written decision.
One thing to check before filing: in some states the board can raise your assessment, not just lower it or leave it alone. California’s appeals boards, for example, are required by law to determine the correct value, which means they can increase it if the evidence supports a higher number. If there’s any chance your property is actually undervalued, think twice or get advice before filing.
When to Bring In a Professional
Most homeowners can handle a straightforward appeal alone, especially when the issue is a data error or a handful of comparable sales. Certain situations favor help.
Property tax consultants specialize in preparing appeals and presenting evidence at the administrative level. Many work on contingency, charging a percentage of your first-year tax savings only if the appeal succeeds. Others charge a flat fee. Fees are negotiable in either arrangement. The limit is that consultants typically can’t represent you beyond the administrative board.
Property tax attorneys can litigate. They can take a case from the initial filing through state court if needed, and they tend to have deeper knowledge of legal arguments around equalization and valuation methodology. For large commercial properties or complex cases with multiple parcels, an attorney who also holds a senior property tax consultant designation can manage the whole process.
The decision usually comes down to dollars at stake. A few hundred dollars a year in potential savings is worth doing yourself. Thousands in potential savings on a high-value property makes a professional’s fee a reasonable cost.