One of the reasons San Francisco has such an acute housing shortage is that its residents can, and often does, block the construction of large residential projects for fear that such developments will change the character of their neighborhoods.

Enter accessory housing units, or DSUs. Often referred to as in-laws or grandma’s apartments, ADUs are small accommodations located on the same land as an existing house and usually have their own kitchens and bathrooms. Development advocates have touted ADUs as key parts of a ‘missing’ solution to California’s housing shortage: they increase density without raising as much concern about changing a neighborhood’s makeup as they are. a large apartment building.

San Francisco has actively pursued the development of ADU. In 2016, the city adopted Ordinance No. 162-16, which legalized the construction of ADUs in all neighborhoods. It was one of the most important of a handful of pro-ADU policies adopted by the city since 2014, starting with a prescription allow landowners to build ADUs if they were also renovating their buildings against earthquake damage – the idea being that renting a new unit would help them recoup the cost of the renovation.

To see how many homes the new ADU legislation added to San Francisco’s available inventory, The Chronicle examined data on permit applications filed with the San Francisco Department of Building Inspection. Since 2014, we have found that 622 ADUs have been built in the city. Of these, almost all – 613 – were built in 2017 or later.

While ADUs are built throughout the city, according to permit data, the types of ADUs built vary by region. In outlying areas of San Francisco to the south and west, like Bayview, Oceanview, and Visitacion Valley, ADUs under construction are more like a traditional grandma’s apartment or in-laws unit. That is, they are units added to the lots of single-family homes.

Closer to the city center and to the north, most of the completed ADUs are on lots that already have three or more units, classified as apartment buildings by the Department of Building Inspection. These ADUs look a little different than the traditional image of ADU people – they tend to be converted basements, garages, and storage areas, rather than new structures in backyards.

But while they may not conjure up the most common image of an ADU, units make up the bulk of the new housing units boosted by the ADU program. DSUs attached to apartment buildings accounted for two-thirds of completed ADU permits and three-quarters of completed new housing units classified as DSUs.

Regardless of their type, the ADUs completed as a result of the city’s new legislation did not significantly increase the city’s housing stock. San Francisco had approximately 406,628 housing units in 2020, according to the U.S. census. Assuming the new ADUs were part of that number, they increased the city’s housing stock by less than 0.2%.

Susie Neilson is a writer for the San Francisco Chronicle. Email: [email protected] Twitter: @susieneilson