Tax appraisals are much lower resolution. They’re *typically* not going to every house to assess quality of finishes, whether you’ve got an end unit of a townhome, how well it shows, and updating the appraisal based on the last three vaguely comparable sales nearby.
They do a big picture snapshot and then use an equation based on information they have on paper to assign a value to each house – square footage above/below grade, bedrooms, bathrooms, plot size. Then they might only revisit the valuation weighting once every 5-10 years.
A mortgage appraisal, OTOH, has someone come out and actually look at the place, then do a deep dive of the last few comparable houses, trying to make sure they’re as comparable as possible. They’re using data that the taxman isn’t, both in terms of how up to date it is and the scope of data available.
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