The difference often comes down to official “living space” versus the total space of the house.
An agent may calculate square footage based on how much living space there is—in other words, areas of the home that are heated such as the kitchen, bathrooms, bedrooms, and so on.
The appraiser, on the other hand, evaluates the total value of a home. That means calculating square footage that includes everything, even an unheated basement, attic, and other nonliving spaces.
This inclusive number should have been recorded by the local municipality when the home was built because it’s used for tax purposes.