Although existing studies uncovered how controversial policies are adopted across American States, little is known as to how regulatory policies are diffused that are technically complex and yet have low political salience. Drawing on existing theories of policy adoption and diffusion, this article identifies the determinants of building code mandates in U.S. state governments. Using original data from state legislatures and administrative agencies from 1980 to 2006, this article finds that both emulation from neighboring states and innovation within the state government increase the likelihood of statewide building regulations. Code adoption follows a hybrid diffusion pattern, whereby external and internal mechanisms occur simultaneously rather than independently from each other.