Housing prices in the world’s most desirable cities are exploding, pricing out even relatively high-income people. Buying a house in Sydney requires a pre-tax income of more than $150,000, according to a study by broker Mortgage Choice. Other capital cities show similar statistics, leaving many residents as lifelong renters. Trailer parks, if developed properly, could address the affordability problem, perhaps more efficiently than any other solution.

Though they’re sometimes the object of ridicule and fear, especially in the US, trailer parks are actually an example of forward-thinking and adaptive zoning. Providing thousands more housing units is often simply a matter of allowing far higher densities than the traditional suburban block, and mobile homes and manufactured housing…