NEW: Townhouses are booming in the US as Americans are desperate for more affordable -- and low-maintenance -- housing options.
-Townhome starts are the highest since 2006
-They're "middle density" housing that is sorely needed in many suburbs
-In major metro areas, townhomes cost ~$100k to $250k less to buy than single-family homes
Townhouses can't solve all the US housing woes, but they are helping a lot in communities smart enough to encourage them.