A lot of real estate is wasted on rooftops; in the 20's, developers started offering penthouses and they became all the rage. At the time, Modern Mechanix's writer was shocked by the prices; the agent responded:
"You ain't seen nothing yet. Several of the new penthouses on Fifth Avenue have sold for $300,000 each. Most of these are sold on the plans, before the building is started, and finished to suit. And we can't finish them fast enough. On the east side alone there are probably 100 under way, on about 65 new apartments. Every building of this type now has from one to four penthouses."
While prices have gone up a bit, attitudes haven't changed at all:
"European architects have been amazed at American disregard for roof tops. For centuries We have used flat roofs merely as rain-and-snow sheds, as places for chimneys to emerge, and more recently as supports for "wireless" wires. At the same time, every large city has for years been crying for more park area.
Some cities, like New York, have even bought up whole tenement districts and torn down buildings to make breathing spaces. Yet the total roof area of a city almost equals that of the city itself, less streets and sidewalks. Enormous areas, estimated at three times the street space, are going to waste today. In future it is safe to predict that these roofs will be covered with homes, playgrounds, gardens, open-air restaurants and even parks. Large buildings now planned are especially suitable for roof dwellings and gardens. Architects now design roofs amply strong enough to support the most elaborate roof dwellings." ::Modern Mechanix