Sleeping box

As part of my exploration of sustainability ideas, I'm contemplating what the minimal requirements for human shelter might be. When I'm camping, I don't need a house. I just need a sleeping bag and a tent. However, if you were sleeping in an urban environment and needed protection from other people as well as from the elements, you would need a solid shelter that could be closed off from the outside. If you were family, you would want to be able to link the shelters together.

For sustainability reasons, the shelter should be as small and inexpensive as possible (least footprint). Perhaps it should also be portable.

What is the smallest, simplest possible (adequate) shelter? What needs must a human shelter meet to be considered adequate? What additional features must it have to provide some minimal comfort?