One of the most useful facts for any appeal: your assessor did not individually appraise your home. With tens or hundreds of thousands of parcels to value every cycle, assessors use Computer-Assisted Mass Appraisal, or CAMA — statistical models that estimate value for whole neighborhoods at once. Those models are efficient, but they make systematic errors you can appeal.
What mass appraisal actually is
A CAMA system builds a model from recent sales in an area, then applies it to every similar property nearby. It uses your property's recorded characteristics — square footage, lot size, age, bed/bath count, quality grade, condition code — and plugs them into formulas calibrated to local sales. The output is a value estimate, not a hand-crafted appraisal of your specific home.
The three approaches inside the model
- Sales comparison — the dominant approach for homes: what similar properties sold for, adjusted for differences.
- Cost — land value plus the depreciated cost to rebuild; common for new or unusual properties.
- Income — for rental and commercial property, value derived from the income it produces.
Why mass appraisal produces errors you can appeal
- It relies on neighborhood averages and can miss what is specific to your property.
- The characteristics on file are often wrong — an overstated square footage, a finished basement that isn't finished, a bathroom that doesn't exist.
- It can't see condition problems: deferred maintenance, a failing roof, foundation issues, or a bad location within the neighborhood (busy road, backing to commercial).
- It applies uniform adjustments that don't fit outlier properties.
- It can pull in poor 'comparable' sales — non-arm's-length transfers, or homes that aren't truly like yours.
How to beat the model
You don't have to prove the whole system is wrong — only that its assumptions are wrong for your property. The two most effective moves are correcting the assessor's data (get your property record card and challenge any overstated characteristics) and presenting better comparable sales than the model used, adjusted to your home. Documented condition problems are powerful because the model literally can't account for them.
Start by requesting your property record card and the comparables the assessor relied on — a public records request compels both. Errors on the record card alone win a surprising number of appeals.
Sources & currency: mass appraisal and CAMA methodology reflect standard assessment practice (IAAO standards) as described by state and county assessors, reviewed July 2026. Specific model factors vary by jurisdiction — request your assessor's methodology and record card for the details that apply to your property.