According to Zillow, Zestimates are created by incorporating "…public and user-submitted data, taking into account home facts, location and market conditions." User-submitted data would include MLS data from Realtors, as well as facts submitted by property owners about their homes. Zillow also uses tax records. But tax records are notoriously inaccurate, especially for older homes that may have been improved over the years without the tax records being adjusted accordingly.