Street-level stores with apartments above them, like these along Main Street in Ossining, NY, are one example of the type of development current federal regulations restrict.

A growing number of Americans wanting to live in walkable, mixed-use neighborhoods—but arcane federal rules make it unnecessarily difficult to build this type of development. A recent study by the Regional Plan Association, released in partnership with LOCUS: Responsible Real Estate Developers and Investors, highlights how—and what lawmakers can do to change it.