House values for properties at specific addresses are available through county tax assessor offices, as SFGate explains. In addition, local real estate agents can often provide this information. Services such as Zillow also provide estimated home valuations as a tool to help determine home value, according to Zillow.
County assessors maintain full databases of properties within each county. Typically, each property's record contains information on the lot size, building size, assessed and appraised values of the property, and the sales history. Those interested in finding home values can do so by visiting the assessor's office, and many counties also make this information available online or over the phone. Tax assessment information is open to the public and free to access, as SFGate points out.
While Zillow doesn't provide official appraisal figures, it uses a proprietary formula to compute estimated home values based on the type of public information available at county assessors' offices. Rather than providing a specific value, Zillow provides a high and low estimated range of a home's value. The estimated value range is a 70 percent confidence interval and varies depending on the data available to Zillow. Homeowners are allowed to provide additional information that isn't necessarily included in the local tax assessor's database; for instance, unless a sale has occurred, the county tax assessor doesn't automatically have information about any remodels that might affect a home's value, as Zillow explains.