There’s unused land even in the most prosperous cities. There are vacant lots that developers hold until the moment is right to build; there are underused parking lots; and there are acres and acres of flat roofs.

Clearly this is not the highest and best use of real estate. The fact that land remains empty represents a failure of the market. There’s no shortage of demand: There are solar companies that would like to put panels on the roofs, priced-out residents who would love to erect mobile mini-dwellings in unused parking lots, and urban farmers who could cultivate zucchini on vacant lots. But often the transaction costs of setting up these enterprises are too high: Landlords don’t even bother investigating the possibility of renting out their unused space because they figure there are bound to be insurmountable complications.

Chief among these feared complications is the potential for lock-in. If you bring in a team of urban farmers to beautify your vacant lot, are they really going to leave when you’re ready to build? Or, if you decide your rooftop garden just isn’t working out, can you really afford to cart off all the soil you hoisted up there?