A property's value can change for many reasons. The most obvious is that the property changes. For example, a bedroom, garage, or swimming pool is added, or part of the property is destroyed by flood or fire. The most frequent cause of a change in value is a change in the market. When market value changes, naturally so does assessed value.
People make value by their transactions in the marketplace.
It is important to remember that the assessor does not create this value. The assessor simply has the legal responsibility to study those transactions and appraise your property accordingly. Just as in many other fields, computers are useful in the assessment process. Assessors are trained to look for relationships between property characteristics and market value. By coding these characteristics and studying sale prices, assessors can estimate value by developing formulas and models.