There are numerous ways in which people make illegal or unauthorized alterations to urban space. This study identifies and analyzes one that has been largely ignored in social science: explicitly functional and civic‐minded informal contributions that I call “do‐it‐yourself urban design.” The research, which began as an investigation into more “traditional” nonpermissable alterations, uncovered these cases—from homemade bike lanes and street signs to guerrilla gardens and development proposals—that are gaining visibility in many cities, yet are poorly accounted for by existing perspectives in the literature. This article examines the existing theories and evidence from interviews and other fieldwork in 14 cities in order to develop the new analytical category of DIY urban design. I present findings on the creators of these interventions, on their motivations to “improve” the built environment where they perceive government and other development actors to be failing, and on the concentration of their efforts in gentrifying areas. This introduces the possibility of conflict and complicates their impact. I argue that DIY urban design has wide‐ranging implications for both local communities and broader urban policy.