AMERICAN suburbs don’t come much more ordinary than Sunrise. In street after street, modest single-storey houses sit behind grass yards dotted with bougainvillea bushes and palm trees. Each house has a mailbox on a pole, with a little red flag to signal when it is full. Sunrise is so similar to other Florida suburbs that when it was built, in the 1960s, the developer lured visitors with a gimmick: an upside-down house, with upside-down plants and an upside-down car in the driveway.

What is being built now seems odder even than that. On the edge of Sunrise, next to the Florida Everglades, eight modernist blocks of flats (the first of them 28 stories high) are to rise, along with offices, car parks and a shopping street including restaurants and a cinema. Erick Collazo of Metropica Holdings, the developer, says the idea is to build a downtown in suburbia. Metropica, as the 26-hectare complex is called, will not really be a downtown. Because of what it suggests about the future of cities and suburbs, it will be more interesting than that.

Florida rivals southern California as the rich world’s most inventive, exuberant urban laboratory. The architectural movement known as new urbanism took off there, producing two settlements that tried to bottle the essence of small-town America: Seaside (the setting for “The Truman Show”) and Celebration, built by the Walt Disney Company. If you want to see out your last years playing golf there is nowhere better: one huge retirement community, known as The Villages, has 48 courses.

Today the fad in south Florida is not golf villages or retro towns but ready-made city centres. Half an hour’s drive south of Sunrise, another Metropica-like development, City Place Doral, is under construction. Two others with even taller towers, Miami Worldcentre and Brickell City Centre, are going up in central Miami. A huge development called SoLe Mia will rise in north Miami. All will combine “walkable” shopping streets, offices and homes—mostly two- and three-bedroom flats in towers. Similar developments have appeared in other American cities, and beyond. But Florida is being overrun.

Builders call these developments “mixed-use”, a term that fails to capture what they are up to. The idea of combining flats, offices and shops even in a single building is not new: look at an old New York district like Chelsea. Metropica and its kin try to create urban cores in places that lack them. Whereas new urbanist settlements often promote a small-town ideal, these sell big-city life, which is why they have words like “metro”, “city” and “centre” in their names. The salesmen claim that residents will be able to live, work and be entertained in a single district.

Ersatz city centres are multiplying now partly because it takes about this long after a financial crisis to begin a big project. Another reason is the rising price of land. Jeffrey Soffer of Turnberry Associates, a big Miami developer, points out that south Florida has almost run out of room to sprawl. Pinched between the Everglades in the west and the Atlantic in the east, it must go up. And although some cities, including Miami, are probably building too many high-rise flats, demand is fairly strong. Foreigners want to own them (most of the people buying flats in Metropica are Latin Americans) and young Americans want to rent them, partly because they find it hard to get mortgages to buy family homes. The towers are growing bigger: 48% of flats constructed in America in 2014 were in buildings with at least 50 units.

Mixed-use development is also being pushed by politicians. They have tweaked the zoning rules that normally separate homes, offices and shops, and allowed taller buildings: Miami approved a more relaxed zoning code in 2009. Rules that require a minimum number of parking spaces to be constructed for every new restaurant table or every 100 square metres of office may also be softened in mixed-use developments. The developers claim, reasonably, that the same parking space can be used by a worker during the day and a diner at night.

As it has done before, Florida is pioneering a new kind of city. Robert Bruegmann, an authority on urban sprawl at the University of Illinois, reckons that American city centres and suburbs are coming to resemble one another. Suburbs are growing denser and more diverse; urban cores are greener, cleaner and often less densely populated than they were (even go-go Manhattan has two-thirds as many people as it did a century ago). Ersatz city centres, which can be built in low-rise suburbs like Sunrise or in built-up areas, bulldoze the distinction further. South Florida is becoming a landscape of scattered centres—sprawl with bumps.

But creating the appearance of urbanity is not the same as making a city. Cities are supposed to be cosmopolitan and surprising; they ought to change in unpredictable ways. Mixed-use developments, by contrast, are fully-formed when they are built—and are too costly for the poor. They are not supposed to be diverse. John Hitchcox of Yoo, a design firm that has worked on Metropica and many other projects, says that mixed-use developments aim to create communities of like-minded people. Though they look like cities, they are supposed to feel like villages.

In fact, the low-rise 1960s suburb where Metropica is being built is already full of cosmopolitan surprise. Behind those monotonous lawns lives a diverse population: one-third of the 88,000 people who live in Sunrise are black and one-quarter are Hispanic. The strip malls are filled with esoteric businesses—a South Indian vegetarian restaurant run by Palestinians, a Vietnamese café, a Dominican hairdresser, even a British shop selling Boddingtons beer and scones. Walkable they are not. But by providing places for immigrants to get ahead, the cheap, ugly, car-oriented strip malls of suburban Florida are already doing what cities are supposed to do.