This Vox Future Perfect article explores how outdated building codes and regulatory requirements create hidden barriers to housing affordability and the construction of “missing middle” housing, such as duplexes, triplexes, fourplexes, and small apartment buildings. Drawing extensively on John Zeanah’s research for the Center for Building in North America, the article examines how regulations governing fire protection, stair design, engineering requirements, elevators, and other building standards can make small multifamily housing financially infeasible. It highlights opportunities for evidence-based code reform that maintains safety while expanding housing choice, improving affordability, and supporting more sustainable community development.