Secondary units (also known as in-law units) are home to tens of thousands of San Francisco residents. Most of these dwellings were constructed within a single family property without city authorization. They serve as a crucial source of housing for low-income households in a city with a significant affordable housing shortage. However, given the vulnerable status of these unauthorized constructions, an estimated 50-100 secondary units are eliminated annually. Efforts in the past four decades to legalize secondary units have failed, but with the current crisis in affordable housing and the exodus of working families from the city, several city legislators are now renewing efforts to bring these units to regulation. Because no mechanism exists that adequately tracks secondary units, it has been unclear what communities are most impacted by this form of housing. ALC’s report takes the first step in identifying these communities to better understand their housing needs and ensure that their voice and presence are brought to bear on future planning and policymaking efforts.