BLDG BLOG's Geoff Manaugh has put up a short piece of design fiction that bears on this: Rentable Basement Mazes.
Within a few years, the market matures.
You can then rent bar cars, home gyms, private restaurants, cheese caves, wine cellars, topless dancing clubs, recording studios, movie theaters, and even an aquarium. You can't sleep in the middle of the night and so you wander downstairs to look at rare tropical fish, alone with fantastic webworks of coral beneath a slumbering metropolis.
Bespoke planetarium cars are soon developed; you step into your own personal history of the sky every night as the clanking metal of distant private rail switches echoes in the tunnels around you, basements unlatching and moving on through vaulted darkness.
Shoe storage. Rare book libraries. Guest bedrooms. Growing operations. Swine flu quarantine facilities.