Accessory dwelling units or ADUs are among the hottest real estate trends in California, providing a possible solution to the state's housing crisis. But according to a new study by Stanford indicates a lot of these so-called 'backyard homes' are being built illegally. Researchers used satellite imagery from San Jose and found that of the 1,300 ADUs built between 2016 and 2020, an estimated 1,000 were built without permits.
Past surveys in the field looking into illegal backyard units have been limited due to sample size in the hundreds. Researchers at Stanford's RegLab randomly chose 15,000 residential parcels. One sample they provided shows images of a backyard in 2016, and then by 2020, a legal ADU added. They used a computer vision model to spot newly constructed ADUs and cross-checked them with official permits.