In Zillow's own words they tell you the following:
The Zestimate® home valuation is Zillow's estimated market value, computed using a proprietary formula. It is not an appraisal. It is a starting point in determining a home's value. The Zestimate is calculated from public and user-submitted data, taking into account special features, location, and market conditions. We encourage buyers, sellers, and homeowners to supplement Zillow's information by doing other research such as:
Getting a comparative market analysis (CMA) from a real estate agent
Getting an appraisal from a professional appraiser
The day has finally arrived! You leave the closing table with the keys
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