An Australian expert tackles the problem of McMansions and affordable housing:

Western suburbs “McMansions” should be converted into apartments to help deal with Perth’s population growth, an expert says.

Leading WA environmental scientist David Kaesehagen said walls or divisions could be built within big properties in the most affluent suburbs to add to housing stock in a rapidly growing market…

The Australian Bureau of Statistics has forecast Perth’s population will grow from 1.8 million to 4.2 million in the next four decades…

Mr Kaesehagen said with the right laws and incentives, owners of large mansions could be persuaded to divide their properties. “This is a way of using existing built form to increase density without introducing high-rise or changing the aesthetic of these established suburbs,” he said.

This is not the first time this has been suggested but I have yet to see a community or significant number of people push for this. I suspect it would be really difficult to do this kind of retrofitting in suburbs often so concerned about property values and density: who would want to be the first resident to have your house subdivided as your neighbors look on and wonder about their housing values? How much would it cost to convert larger homes into multiple units when they were originally intended for single families? This might work best in McMansion neighborhoods that are abandoned or not yet opened (each of these would pose their own set of problems) so there would be no community members in opposition. But, then why not build higher density developments in the first place?