The social determinants of health refer to social, economic and environmental factors that influence well-being including economic inequality, residential segregation, sub-standard housing, lack of supermarkets, schools, transit and open-space, and disruptions to family and social networks. This paper asks whether and how the practice of health impact assessment (HIA) can integrate the social and physical determinants of health into planning processes, overcome institutional and analytic barriers for health analyses in environmental impact assessment, and offer a new model for healthy urban planning. This is done by examining how a municipal health agency, the San Francisco Department of Public Health, utilized HIA to conduct health analyses of development projects, collaborate with other city agencies and community groups, and initiate a multi-stakeholder prescriptive HIA all aimed at integrating health into environmental planning practices. This case is important because the San Francisco DPH is the first city agency in the US to experiment with using HIA that aims to capture the physical and social environmental health impacts of projects and plans. The paper finds that HIA can inject the social determinants of health into planning when public agencies embrace an expanded definition of environmental health, organize health advocacy networks within and outside government, and generate a broad scientific evidence base to substantiate policy change.