They are compared to “comparable” properties that have recently sold. This can be very difficult if the property is unusual or in a neighborhood where sales are infrequent. Usually from 3-4 comps they make a comparison table that assigns a value to the difference in each parameter, maybe having another bathroom is typically worth $5K, and that gives a group of 3-4 estimates. Then it’s a policy thing to weight and average the numbers to get the official appraisal.
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