I am constantly asked by individuals, "what I think of some of the valuation websites that exists on the internet today (Zill**, HouseVal*** etc...)". If you're trying to determine a value for a residential property in an older, fully developed city (L.A., San Diego, Santa Monica), the property values you get from these sites can be completely inaccurate.
The foundation of these web valuation sites is based on gathering recent sales in the neighborhood with similar livable sq.ft. and bedroom/bathroom count. With all other amenities being equal. Some sites allow for "any" individual to go in and add additional amenities or improvements. Aside from the tax accessors records, their are no checks and balances on the validity of this information. Most importantly, there is no process that exists by a trained, experienced individual to interpret how the local market reacts to such improvements, repairs or alterations ( how much a positive or negative element is in terms of dollars).
Many of the homes built pre 1935 were not built with the tract home mentality. Therefore you have many custom homes that have "value added" amenities that change from house to house. For homes built post 1935, they may have been built with the tract home mentality, but there has been substantial customization and/or alterations done to the homes throughout the years.
A person leaves one of these websites with an inaccurate or extremely broad valuation range. For whatever reason the individual is curious about the house value (purchase, sale, refinance, home improvement etc...), having an inaccurate value can be very costly when you're dealing with an asset that is worth several hundreds to millions of dollars. Any lender is going to require you to have an exact, verifiable number anyway.
Bottom line, one can get just enough information from one of these home valuation websites to be dangerous!