Question: how to house the 9 billion people of 2050, when today 1 billion people are homeless or living in bad shelters? Answer: in every sustainable way possible, depending on culture, climate, environment and other local conditions.
Homelessness is mainly a majority world challenge, but millions are homeless in Europe and North America too. In these regions, empty houses and flats are available, but the homeless are too poor to buy or rent them and cannot get credit. Yet housing policy is tricky; the recession of the first decade of this century was partly caused by irresponsibly offered housing credit.
Other policies besides appropriate credit will involve reclaiming and retrofitting the derelict housing stock of urban centres, helping people build their own homes, developing new forms of group housing, and putting housing where people can use it conveniently and safely without cars. All this will need clever financing and public/private partnerships
Some old forms of housing are becoming modern. Mud has been a building material since before the Iron Age; it is now a building material for the rich in some parts of the US. It, and new forms of mud bricks, are being “redeveloped” for building use in much of Asia and Africa. Recycled glass and plastic are being used elsewhere, and wood is making a strong return. The challenge is not building the houses, but getting people into homes.