Today, we take for granted the wisdom of renovating old factory buildings into malls or condos, of making once decaying waterfronts into vibrant public spaces, of protecting historic buildings under landmark laws, and of building public housing on a human scale rather than as high-rises. In contemporary cities, it is now common for community groups to plant gardens in empty lots and to buy abandoned apartment buildings from the city for a dollar and fix them up.

But these and other urban planning policies and practices have not always been accepted. Before they became widespread, they...