Ours is the century of the city. For the first time in history, more than half of the people in the world, 3.3 billion of us, live in cities. By 2050, according to the best projections, urbanites will account for as much as 70% of the global population.

Over the next 50 years we will spend trillions of dollars on city building. The question is: How should we build? For many economists, urbanists and developers, the answer is simple: We should build up. But the answer is more complex...