How Idaho Assesses Property: Market Value, Mass Appraisal and Ratio Studies

Every Idaho property tax bill starts with one number the owner did not choose: the assessor's value. The local government module showed what happens after that number exists (Property Tax Math). This lesson is about where the number comes from, and how the state checks whether the county got it right.

The legal target: market value on January 1

Idaho Code 63-205 says all real, personal and operating property "must be assessed annually at market value for assessment purposes as of 12:01 a.m." on January 1. Read that sentence slowly.

• The target is a sale price. The assessor is estimating what a willing buyer would pay. What the owner paid, what it would cost to rebuild and what it is worth to the family living there are separate questions.
• The date is fixed. A house that burns in March or gains a garage in May is valued as it stood at 12:01 a.m. on January 1. (Damage and new construction get their own rules and rolls.)
• It happens every year. Values move with the market even in years nobody visits the property.

Some property is valued on a different basis by law. Farmland is valued by what it earns as farmland, timber by its productivity, and rate-regulated utilities were valued by the Tax Commission until 2026. Lesson 4 covers those. For homes, shops, offices and vacant lots, market value is the rule.

Mass appraisal: valuing a county at once

A county with 40,000 parcels cannot hire a fee appraiser for each one every year. So assessors use mass appraisal: they group similar properties, study what similar ones sold for, build cost and income models, and apply those models across the group. A fee appraiser asks "what is this house worth?" A mass appraiser asks "what are houses like this worth, and how does this one differ?"

Idaho Code 63-314 sets the pace. Every taxable parcel must be physically appraised at least once every five years, with minimums along the way.

Parcels not appraised that year are indexed, which means multiplied by a factor drawn from sales and the year's appraisal results. If two-bedroom ranch homes in one neighborhood sold for about 7 percent more than last year's values, the unvisited ones in that group may get a 1.07 factor. The statute also requires a written plan for the cycle, approved by the Tax Commission, and lets county commissioners levy up to 0.04 percent of market value for a property valuation fund to pay for it.

The state's check: sales ratio studies

Once a year the Tax Commission tests each county's work with a sales ratio study. Under IDAPA 35.01.03, Rule 131, it compares assessed values for January 1 with time-adjusted sale prices from October 1 of the prior year through September 30 of the tested year. Each sale produces an assessment ratio: assessed value divided by price. The study is done in March, and Idaho Code 63-109 requires notice of any improper assessment by the first Monday of April.

For residential, commercial and other non-farm categories, the rule measures the level of assessment with the median ratio. The standard is 90 to 110 percent. A median of 0.95 means the county's values sit at about 95 percent of market. Fine. A median of 0.82 means values lag the market, and that category is paying less than its share compared with categories assessed closer to 100 percent.

The rule adds a statistical safeguard. Under Rule 131.11, the Tax Commission finds "reasonable statistical certainty" that a category is out of line only if:

• the median is below 90 or above 110 percent and a 90 percent two-tailed confidence interval around it fails to include 90 or 110 percent, or
• an 80 percent confidence interval fails to include 90 or 110 percent for the current study and the two before it.

If a category fails, the Tax Commission may order the county to adjust it at its equalization meeting. The adjustment factor is 100 percent divided by the median.

Level and uniformity are separate questions. A county can have a perfect 100 percent median while assessing cheap homes at 120 percent and expensive homes at 80 percent. Those errors cancel in the median and do not cancel on anyone's tax bill. That is why appraisers also look at the COD and at the price-related differential (the mean ratio divided by the weighted mean ratio). Rule 131 adopts the International Association of Assessing Officers' "Standard on Ratio Studies" by reference, and that standard is where those uniformity measures and their target ranges live. In the example, the mean ratio is 0.878 and the weighted mean is 0.875, for a differential of 1.004: no sign that pricier homes get a break.

The notice in June

Idaho Code 63-308 requires the assessor to mail the valuation assessment notice no later than the first Monday in June (June 1 in 2026). As the earlier module explains, the notice since 2024 must show two prior years of market value, two prior years of tax by district, the percent change for each district, each district's budget hearing date and a phone number. Read it for these items:

• Market value this year and the last two. A jump far above the neighborhood is worth an appeal (lesson 2).
• Exemptions shown. If you live there and the homeowner exemption is missing, call the assessor now, not in December.
• Land and improvement values separately. A land value that tripled while the house barely moved points to a land model change for your area.
• The hearing dates. Budgets, not values, set the size of the tax. The notice tells you when each board decides.