How your property tax assessment value is determined comes down to a local official called the tax assessor, who estimates what your property would sell for and then applies a set of adjustments defined by state and local rules. That estimate, produced on a schedule your state sets, becomes the anchor for everything that follows on your tax bill.
Who Sets the Value and How Often
Every parcel in a jurisdiction sits in the assessor’s records, and the assessor’s office is responsible for keeping an estimated market value attached to each one. Most states require reassessments on a schedule ranging from every year to every five years. A handful allow gaps of up to ten years, and nine states have no fixed reassessment requirement at all.1Tax Foundation. State Provisions for Property Reassessment
Between those reassessments, your value usually stays flat. Two events break that pattern: pulling a building permit for work that changes the property, and a sale that transfers ownership. Either can trigger an update outside the normal cycle.
What the Assessor Looks At
The assessor’s file on your property tracks its physical characteristics. Livable square footage. Lot size. Number of bedrooms and bathrooms. Finished spaces like attics or basements. Outbuildings. Each of these feeds the valuation, and each is a place where recorded data can drift out of sync with reality.
Age and condition adjustments come next. Older structures often receive depreciation adjustments that lower the value, while recent renovations push it higher. Location factors matter too: proximity to good schools, parks, and transit adds value, while flood zones or environmental contamination can subtract it. Zoning classifications play a role as well, since land approved for denser development generally carries a higher valuation than land restricted to single-family homes.
The Three Valuation Approaches
Assessors use one of three methods to translate all that data into a dollar figure, and the right method depends on the type of property.
Sales Comparison
For most residential properties, assessors rely on the sales comparison approach. They look at nearby homes with similar features that sold in the past six to twelve months and use those prices to estimate what your property would sell for. This is why a run of high sales on your street can move your assessment upward at the next cycle even if you’ve done nothing to your own home.
Cost Approach
When a property is unusual, or there aren’t enough recent sales nearby to build a reliable comparison, the assessor may use the cost approach. This method estimates what it would cost to rebuild the structure from scratch using current material and labor prices, then subtracts depreciation for the building’s age and condition. The result is treated as the property’s value.
Income Approach
Commercial and rental properties are more commonly valued through the income approach, which focuses on the revenue the property generates. The assessor estimates annual rental income, subtracts operating expenses, and applies a capitalization rate to arrive at a value. A single-family home you live in almost never gets valued this way; a small apartment building or a strip of storefronts almost always does.
Mass Appraisal
For jurisdictions managing tens of thousands of parcels, all three approaches feed into computer-assisted mass appraisal systems that apply statistical models to large groups of similar properties at once.2International Association of Assessing Officers. IAAO Standard on Mass Appraisal of Real Property These systems are efficient but imprecise by design. They work well on average and can miss the mark on individual properties, which is the practical reason two houses that look nearly identical can end up with meaningfully different assessed values.
From Assessed Value to the Number You Owe
Your assessed value is not your tax bill. Two more numbers sit between the assessment and the amount you actually owe: the assessment ratio and the millage rate.
Many jurisdictions tax only a percentage of market value rather than the full amount. If your home has a market value of $300,000 and the local assessment ratio is 80 percent, your taxable value is $240,000. Some states assess at 100 percent of market value, while others go as low as 10 percent. The ratio itself doesn’t make your taxes higher or lower; it just shifts where the math lands.
The millage rate, sometimes called the mill levy, is the tax rate your local governments apply to that taxable value. One mill equals one dollar of tax for every $1,000 of assessed value. If your taxable value is $240,000 and the combined millage rate from your county, school district, and city is 25 mills, the calculation is $240,000 ÷ 1,000 × 25 = $6,000 in annual property tax.
That formula also tells you how much a change in the assessment is worth in real dollars. Knocking $30,000 off your assessed value at a 25-mill rate saves you $750 a year, every year, until the next reassessment.
Where To See the Value and the Data Behind It
Your assessment notice arrives by mail once the assessor completes the valuation cycle, and in many jurisdictions it’s also posted online. A typical notice includes the current assessed value, the prior year’s value, the property’s legal description, and the deadline to file an appeal. Contents vary, but those four items are the ones to look for.
To see what data the assessor actually used, request your property record card from the assessor’s office or their website. The card shows recorded square footage, lot dimensions, year built, number of rooms, and any noted improvements. Errors here are more common than most people realize: an extra bathroom that doesn’t exist, a finished basement that’s actually unfinished, or square footage inflated by a few hundred feet can each add thousands to your assessed value. If you spot a factual mistake, some assessor offices will correct it informally without requiring a full appeal.
When the Value Looks Wrong
If the record is accurate but the value still seems high compared to what nearby homes are actually selling for, the assessment can be challenged. Every jurisdiction has an appeal process with a deadline printed on the notice, and appeal windows can be as short as 25 days or as long as 90 days from the notice date. A few states measure the deadline from a fixed calendar date instead. Miss the window, and you’re locked into the current assessment for the tax year; in states with multi-year reassessment cycles, that can mean waiting three to six years for another chance.
One boundary worth knowing before filing anything: in many jurisdictions, the review board that hears your appeal has the authority to raise your assessed value if it concludes the assessor undervalued the property. It doesn’t happen often, but it’s a real possibility for properties that have been significantly improved without the assessor’s knowledge. A successful appeal, when it goes your way, typically reduces the assessed value by 10 to 15 percent and carries forward until the next reassessment.
A review board can only adjust the assessed value. It cannot change the millage rate, the assessment ratio, or how your local government spends tax revenue. And if market values in your area are genuinely rising, your assessment may climb back up at the next reassessment regardless of this year’s outcome. Understanding how the value was built in the first place, from the property record through the valuation method to the ratio and millage, is what tells you whether the number on the notice is worth challenging.