This study explored the relationship between social capital and aspects of the built environment, focusing in particular on the walkability of suburbs as determined by street network design and the mix of land uses. We measured social capital and feelings of personal safety in 335 residents of three suburbs in metropolitan Perth, WA, and collected objective and perceived data on the built environment. After adjustment for demographic factors, the built environment was found to have a significant but small effect on social capital and feelings of safety, particularly in relation to the number and perceived adequacy of destinations. A high level of neighbourhood upkeep was associated with both higher social capital and feelings of safety.