There exists an almost absurdly simple fix that could help ease the housing crisis. It would cost the government nothing, require deleting just five words from a 50-year-old federal law, and has enjoyed quiet support from housing researchers and leaders for decades.

The target is an obscure regulation that requires every manufactured home to be built on a “permanent chassis” — a steel trailer frame that can attach to wheels. The idea was that the chassis was necessary — even after the home was installed and the wheels taken off — because manufactured houses, which trace their roots to World War II trailers, could theoretically...