Using Remote Sensing to Estimate the Population of Unpermitted ADUs in San José

In recent years, in an effort to address a severe housing shortage, California has tried to make building legal accessory dwelling units (ADUs) as easy as possible. Official records show that property owners in San José, one of the largest cities in northern California, legally built about 291 detached ADUs from 2016 to 2020, which represents a large increase compared with previous years. But a new study suggests that more than 1,000 unpermitted ADUs were constructed in the same period—reflecting the state’s severe housing shortage and underscoring the urgent need for affordable housing options.

With support from the Housing Solutions Lab, a team of Stanford University and Massachusetts Institute of Technology (MIT) researchers used a computer vision model, human annotations on a stratified sample of parcels, and publicly available satellite imagery to estimate the prevalence of unpermitted ADUs. They found that unpermitted ADUs are most likely to be located in dense, diverse, overcrowded communities.

The team’s findings suggest that even after California’s 2016 policy changes, legal ADUs are still hard to build. Previous research suggests that homeowners have to navigate a complex set of rules and regulations before they can add ADUs to their properties. Cost may be a barrier as well; in San José, fees related to permit issuance, plan review, and inspections currently range from approximately $3,000 to $4,700. Such fees may inadvertently push people to develop informal ADUs rather than going through existing official channels, according to the authors.