Mansions for Munchkins
These tiny houses, from the Tumbleweed Tiny House Company
, make all the in-law units and cottages I've ever seen seem like palaces. The smallest one is 50 square feet. From the description of the XS model: Because the exterior dimensions of this structure are well within the legal limits for travel on U.S. roadways, the XS-House can be taken almost anywhere. The porch and awning fold up for added convenience.
Folding porches! Sadly, they don't really seem Manhattan-friendly, the one place I'd see this being really useful.