Sustainable urbanism attempts to curtail the negative environmental consequences of urban sprawl through best practices in urban design, yet there is a perception among experts that many developments may fall short of ideals in practice. Studies have enumerated multiple implementation barriers as evidence of shortcomings, yet few studies have empirically linked barriers to environmental impact. To address this gap, this study asks: how do institutional structures constrain the capacity to implement design alternatives and achieve the environmental goals of sustainable urbanism? The study presents interview, document, and environmental performance data in an institutional analysis of a case study, Civano, a sustainable planned development in Tucson, Arizona, to characterize implementation barriers and connect them to environmental performance outcomes. It finds that the confluence of national alternative energy discourses, local land-use and other formal regulatory conflicts, and tensions with informal real estate development industry norms created conditions that generally favored building technology solutions to reduce water and energy consumption, over site designs to achieve broader environmental goals. The findings suggest that novel institutional arrangements will be required to encourage private sector implementation of sustainable urban designs.