Dharavi, Mumbai, where population density reaches 1m people per square mile

In 1983, architect Peter Calthorpe gave up on San Francisco, where he had tried and failed to organise neighbourhood communities, and moved to a houseboat in Sausalito, a town on the San Francisco Bay. He ended up on South 40 Dock, where I also live, part of a community of 400 houseboats and a place with the densest housing in California. Without trying, it was an intense, proud community, in which no one locked their doors. Calthorpe looked for the element of design magic that made it work, and concluded it was the dock itself and the density. Everyone who lived in the houseboats on South 40 Dock passed each other on foot daily, trundling to and from the parking lot on shore. All the residents knew each other’s faces and voices and cats. It was a community, Calthorpe decided, because it was walkable.

Building on that insight, Calthorpe became one of the founders of the new urbanism, along with Andrés Duany, Elizabeth Plater-Zyberk and others. In 1985 he introduced the concept of walkability in “Redefining Cities,” an article in the Whole Earth Review, an American counterculture magazine that focused on technology, community building and the environment. Since then, new urbanism has become the dominant force in city planning, promoting high density, mixed use, walkability, mass transit, eclectic design and regionalism. It drew one of its main ideas from the houseboat community.

There are plenty more ideas to be discovered in the squatter cities of the developing world, the conurbations made up of people who do not legally occupy the land they live on—more commonly known as slums. One billion people live in these cities and, according to the UN, this number will double in the next 25 years. There are thousands of them and their mainly young populations test out new ideas unfettered by law or tradition. Alleyways in squatter cities, for example, are a dense interplay of retail and services—one-chair barbershops and three-seat bars interspersed with the clothes racks and fruit tables. One proposal is to use these as a model for shopping areas. “Allow the informal sector to take over downtown areas after 6pm,” suggests Jaime Lerner, the former mayor of Curitiba, Brazil. “That will inject life into the city.”

The reversal of opinion about fast-growing cities, previously considered bad news, began with The Challenge of…

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