The architects at Katz Chiao—Terri Chiao and Deborah Grossberg Katzhave—bulit a treehouse and a cabin inside of an apartment in Bushwick! They told FastCoDesign that it cost just $2,000, and, “as a result, living in the space can feel like living outdoors, in a tiny community of two houses." The project transformed the one room loft into a 3 room compound... with room for a cat, no less.
On their website, they explain the idea further: "Conceived of as houses within houses, the cabin and treehouse serve as private sleeping cabins, each with its own semi-private garden set off from the shared living space. The treehouse is lofted above the ground and houses a study and storage area below. The cabin has a storage space built into the raised floor. A garden and a place to hang clothes occupy the small semi-private area that is formed between the cabin entrance and the windows."
The structures were built in 2009, but it looks like it's still around since new photos have popped up on Flickr some children are living in... without roommates.