Zillow Expands and Improves Database of Homes

This morning, thanks to an entirely new Zestimate algorithm, we’ve expanded and improved our living database of homes. We’ve added Zestimate home valuations to more than 25 million new homes and improved Zestimate accuracy across the country. We now have data, Zestimates and Rent Zestimates on about 100 million homes in the U.S.

Over the past five years, Zillow users have submitted information on more than 28 million homes, adding updates about remodels or home facts. This gives us an inimitable database of almost every home in the country. Our new Zestimate algorithm better takes advantage of this database and of our unique home profiles, incorporating more data points and new modeling approaches.

The new algorithm is producing Zestimates with significant improvements in accuracy. Nationally, our new median error is 8.5%. That’s 33% more accurate than what our median error with the old algorithm would be during the same time period. Several large markets — Denver, San Diego and Washington DC — have median errors under 6%.

Of course, we continue to compute and publish Zestimate accuracy, down to the county level, every three months. Since so much of Zillow’s mission is about increasing the transparency of data in the real estate market, we apply this same rule to our own data and, by doing so, help consumers and professionals better understand the performance of our models across all geographies. We hope the improved and expanded set of Zestimates will prove a useful starting point for consumers seeking to determine a home’s value. To see accuracy in your county, go to our Zestimate accuracy page.

For more in-depth details about the new Zestimate algorithm, visit the Zillow Research Page and read my brief there.

UPDATE: We’ve noticed questions in Zillow Advice about why we’ve redone the Zestimate history. In short, it was important to do because we want changes in Zestimate values over time to reflect changes in the market, not changes in how Zillow computes its algorithm. Our new algorithm doesn’t just improve current accuracy, it improves historical accuracy, giving consumers a better idea of what a home was worth over time.

Thanks for your questions and feedback.

Dr. Stan Humphries is a real estate economist and real estate expert for Zillow. Stan is in charge of the data and analytics team at Zillow, which develops housing market data for most major metropolitan statistical areas in the U.S., and provides economic research for current real estate market conditions. He helped create the algorithms for the popular Zestimate® home value and the Zillow Home Value Index (ZHVI).