Accessory dwelling units, or ADUs, are among the hottest real estate trends in California, offering a potential solution to the state’s housing crisis.
But according to a new study from Stanford, many of these so-called “backyard homes” are being built illegally. Researchers used satellite images from San Jose and found that of the 1,300 ADUs built between 2016 and 2020, an estimated 1,000 were built without permits.
The research aims to address equity issues as some builders work in the shadows to create homes for those who need them most.
A construction worker who asked to have his identity protected for fear of losing his contractor’s license said he continues to see illegal housing being built from the Peninsula to the South Bay and beyond.
“They’re remodeling garages. They’re doing rear additions. They’re putting in separate entrances. They’re doing what they can to accommodate their families,” the contractor, who gave his name as “William,” told CBS. Bay Area News.
William has been building houses for decades. But with sky-high prices, lengthy permitting and inspection processes, to name just a few housing barriers, he receives requests that put his small business in a difficult position.
“I have people calling me all the time asking, ‘Can you do a unit for me?’ But I really don’t want to jeopardize my license,” William said.
The dozens of homes that William has built have superstructures with the necessary permits. But he said he has built at least three unpermitted units to create housing that he rents out below market.
“The amount of time it takes to get a permit, get plans done, go through the planning, the whole process is so time consuming,” William said.
Using satellite imagery, including Google Earth, and other assessment methods, researchers at Stanford University’s Regulation, Evaluation, and Governance Lab found that for every legal accessory dwelling unit built in San Jose between 2016 and 2020, there were three to four “informal ‘ homes were. or unpermitted ADUs built.
Andrea Vallebueno is a data scientist and research fellow at Stanford University’s RegLab.
“We think it offers an innovative method that can be developed on a large scale, because it’s so much easier to look at these kinds of images than it is to do that kind of fieldwork,” says Vallebueno.
Previous field studies of illegal backyard units have been limited due to sample sizes in the hundreds. Researchers at Stanford’s RegLab randomly selected 15,000 residential plots.
One example they provided shows images of a backyard in 2016, and in 2020 a legal ADU was added. They used a computer vision model to spot newly built ADUs and checked them against official permits.
“If that model can be replicated across California, we think it is a strong and responsible way to use modern technologies to improve housing outcomes,” Ouyang said.
Derek Ouyang is part of the RegLab and says the unauthorized detections will never be shared with anyone outside their team to protect residents’ privacy.
They found that most of these informal ADUs are being built in dense communities of color, with lower income levels.
He hopes policymakers will gain a better understanding of the prevalence of illegal construction so they can design better programs to help builders, as well as renters.
“These are the types of policy solutions, whether it’s legalization programs or amnesty programs, or just lowering fees that can help reduce the inequalities that come with this,” Ouyang said.
“These things should be streamlined. They should actually have one representation from the county or city to be able to look at your plans and say, ‘Hey, this makes sense, let’s approve this instead of taking months to do it. and maybe even a year,” said William .
Homebuilders like William hope that permitting rules will be adjusted and improved to make the process of building above board more efficient, ultimately helping families in need of affordable housing.
“A lot of these people are putting their families there to make ends meet with the high housing prices. What’s a person to do?” William asked.
He would rather build a 100% legal ADU, but the reality is that many are still being built in the shadows. Official statistics, including those from the state Housing Department, do not account for unauthorized units.
Without that, researchers believe housing policies, including ADU liberalization laws since 2016, risk being potentially misleading.
They say efforts to boost housing growth will not actually expand the net housing market, but rather worsen inequality if the benefits flow mainly to more affluent households.
Their research will likely be published next week in the Journal of the American Planning Association.
