How skinny can you build? Cities and first-ring suburbs are making a decisive comeback in the new economy, but building in old-growth neighborhoods and emerging corridors has its inevitable constraints, particularly if you are weaving detached homes back into the neighborhood fabric. One common challenge is the narrow lot. In some cases, such zoning is a consequence of tradition; in others, it is the result of privately subdivided properties or municipal smart growth initiatives that offer fast-track permitting for higher density redevelopment. Regardless of the reasons, here are a sampling of 20-feet-wide homes that live quite comfortably in close quarters.