Your Assessment Notice Is Not a CMA

Every June, assessment notices land in Ada County mailboxes, and every June we have the same conversation twice a week in two different flavors.

One version: "The county says it's worth $610,000, so that's what we should list at."

The other: "The county says it's worth $610,000 and there's no way — I'm appealing."

Both treat the assessed value as a verdict on market value. It isn't, and understanding why is worth more than any single number on that notice.

What the assessor is actually doing

Let's be fair to the county first, because the sloppy version of this argument gets it wrong.

Ada County is not guessing, and it is not aiming at some artificial number. Idaho Code § 63-205 requires property to be "assessed annually at market value for assessment purposes." The Idaho State Tax Commission describes the job this way: "Each year your county assessor's office must estimate how much a typical buyer would pay for your property as of January 1. It's assessed at 100% of market value less any exemptions."

And the state checks the work. The Tax Commission runs an annual ratio study, and under IDAPA 35.01.03.131 a county's assessment level is considered to be market value only if the ratio study's measure of central tendency falls within 10% of 100%. Counties outside that band can be ordered to adjust.

Ada County passes. In the most recent study we could verify — 2024 assessments, studied in March 2025 — Ada County's residential median ratio was 92.21%. In compliance, and running roughly 8% below market.

So the assessor is trying to hit market value and is graded on it. Which makes the divergence more interesting, not less.

Three structural reasons the numbers differ

1. The date. Idaho assesses as of 12:01 a.m. on January 1. Notices go out by the first Monday in June. And the sales used to build those values are older still — the Tax Commission's own Ratio Study Manual notes that sale prices "typically will reflect market values as of July 1 of the prior year," and instructs assessors to time-adjust them forward to January 1.

Do the arithmetic on a notice arriving in June 2026: a January 1 valuation, built substantially from 2025 sales. You are listing into a market that is somewhere between six and eighteen months ahead of that number. In a flat market that's noise. In a moving market it's the whole story.

2. Mass appraisal is designed to be right in aggregate, not on your house. This is the part the state says out loud, and it's the most useful sentence in this entire post. From the Ratio Study Manual:

"Although the ideal ratio is 100%, in practice it is rarely possible to precisely predict the selling price of individual properties."

And:

"In mass appraisal aiming at market value, we expect approximately equal numbers of properties to sell for more or less than their assessed values."

That is a formal statement that individual error is expected in both directions. An assessment can be correct as a system output and wrong about your specific house at the same time. Those are not in conflict.

3. Condition and finish. The assessor is valuing tens of thousands of parcels from construction cost data, sales, and recorded characteristics. Ada County explicitly invites owners appealing a value to submit "a realtor's comparative market analysis, copies of independent appraisals… repair estimates" — which tells you that condition-specific information is something the county expects to receive rather than something it already holds.

The county's own framing is the honest one: "The assessor does not set a value for your property. He or she just estimates what a typical buyer would reasonably pay for it on January 1st."

An estimate of what a typical buyer would pay, six months ago, for a house with your characteristics. That's a genuinely useful number. It is not a pricing strategy.

The other half: your value went up, your taxes may not

The most persistent misconception in Idaho property tax has nothing to do with market value.

Ada County states it directly: "It is a common misconception that property tax burdens change directly in proportion to property values." And: "Idaho's property tax system is budget-driven, not rate-based."

The mechanic is simple once you see it. Taxing districts set budgets. The portion funded by property tax gets divided by the total taxable value of everything in the district, and that produces the levy rate. When total value rises and budgets are capped, the rate falls. Under Idaho Code § 63-802 a district's property tax budget is generally limited to 3% over its highest certification in the preceding three years, plus new construction and annexation growth.

A 15% assessment increase in a year when everyone's value rose 15% and budgets rose 3% is not a 15% tax increase.

Two things to check that most people don't

The homeowner's exemption. Idaho Code § 63-602G exempts the lesser of $125,000 or 50% of market value on an owner-occupied primary residence, plus up to one acre. Ada County treats it as one-time — apply once, unless ownership changes or you move.

Something important changed here in 2026. House Bill 843, retroactive to January 1, 2026, eliminated proration of the exemption. The Tax Commission's guidance: "There is to be no proration of the exemption, regardless of when the application is filed during the calendar year." If you bought mid-year and were told you'd only get a partial exemption based on a filing-date schedule, that schedule is gone. Some published county material still shows the old prorated table.

The circuit breaker, formally the Property Tax Reduction program. For 2026, the income limit is $39,130 and the benefit runs $250 to $1,500. Eligibility requires being 65 or older, blind, widowed, disabled, a former POW or hostage, or a fatherless or motherless child under 18 — plus ownership and occupancy. Applications run January 1 through April 15. There's a value cap: you're disqualified if your assessed value exceeds the greater of $400,000 or 200% of the county median for homes receiving the homestead exemption, though 100% service-connected disabled veterans are exempt from that cap.

If you do want to appeal

Appeals go to the Board of Equalization and must be filed by the fourth Monday in June — the county cuts off at 5:00 p.m., filed with the Ada County Commissioners' Office. Start with an informal review with the deputy assessor first.

And note what the county tells you to bring: "The best evidence is typically sales data from the marketplace, written analysis from a Realtor or other professional source." A comparative market analysis is admissible evidence in a tax appeal. It's just not the same document as the notice.

General information about how Idaho property tax works, not tax or legal advice. For your specific situation, talk to the Assessor's office or a tax professional.

Sources: Idaho Code §§ 63-205, 63-308, 63-501, 63-602G, 63-705, 63-724, 63-802 · House Bill 843 (2026) · Idaho State Tax Commission Ratio Study Manual and Property Tax Reduction guidance · IDAPA 35.01.03.131 · Ada County Assessor