In many old buildings, with high ceilings, it is easier to build a house inside than to add new walls.
Lots of young families need an intimate space when they go and visit their parents in summer. Others look for a smaller room where they can work and find refuge from kids noises. A room they can assemble and disassemble easily by themselves, and that they can move around other houses or when they emigrate to some far place. An ambulant family home, that can be passed from generation to generation, becoming the relative center of the descendants. The tiniest legacy.