Co-organized with Generation Investment Management, this meeting focused on the built environment while further developing ideas from our May 2020 and July 2020 Complexity of Sustainability meetings. How do design choices and developmental decisions related to the built environment moderate the sustainability of a building, factory, city, society, ecosystem, or ultimately, the entire planet? This is an inherently complex question, and the answer depends on the interactions between the built environment and multiple socio-economic, technological, and natural complex systems. In many of these areas, our understanding of the bigger issues is still in its infancy. There is an urgent need for developing a systemic scientific framework for addressing such questions in order to reduce our species’ environmental footprint and ensure a livable planet. This meeting highlighted applicable areas of current complexity research and explored our understanding of the built environment's impact on sustainability at multiple scales. The meeting’s goal was for scientists to engage and inform applied decision-makers and practitioners about the underlying science as we work towards creating a more sustainable society.