On-line home estimate sites don't work. Just ask the homeowners who filed a class-action lawsuit against Zillow over their Zestimates.
The problem begins with the fact that each home is different. Homes aren't like stock shares. Each share of Apple stock, for instance, is exactly the same as every other share of Apple stock.
Homes, on the other hand, are each completely different, regardless of whether they were built by the same builder and have the same plan.
The major problem with all automated home estimate sites is they are limited to the data on hand. Typically, that includes only square footage, number of beds and baths, recent sales prices and assessor tax values.
What is not included are location nuances such as is the property on a quiet street or next to a main thoroughfare. Did the owners remove that cheap carpeting and replace it with hardwood floors? Were the appliances replaced with high-end hardware? Granite countertops or tile? Dual-pane windows to replace single-pane windows.
The list goes on.