Often called a “house-within-a-house,” today the method is not that extreme, and it does not mean two separate structures. Modern strong materials, and clever engineering, make it fairly easy. For instance the same set of rafters supports the outer shell roof as the inner shell ceiling. The basement and attic serve as the separation on the bottom and top of the structure. A glass sunroom, aptly called a “conservatory” in the past, separates the shells on the south, so only the double north wall stands out as different.
Many commercial buildings
use the double shell concept. In fact, some commercial double-shell buildings use the temperate outer shell space as a lobby.