U.S. suburbanization has led to significant environmental and social challenges.

This paper examines the contribution of neighborhood design to these outcomes, focusing on the role of “Garden City Design” (GCD)—a prominent design paradigm used to develop American suburbia. I introduce a methodology to quantify the key attributes of GCD using neighborhood layouts, which allows me to assess the national prevalence and explore the consequences of this form of design. Combining this novel measure with human mobility and emission data, I estimate the implications of GCD on neighborhood outcomes using an Instrumental Variables (IV) approach that leverages the phased national adoption of GCD over time. My results show that GCD raises neighborhoods’ greenhouse gas emissions by 0.54 metric tons per person yearly, triples social isolation, and adds 86 daily sedentary minutes. Overall, the prevalence of GCD accounts for 30% of the adverse effects linked to suburbanization, underscoring the critical role neighborhood design holds in urban sustainability.