When I was five, four of us lived in a one car garage for a year. A sink in the corner with a
pitcher pump, a wood burning stove, a hide-a-bed couch for the folks, and bunk beds for sister and me.
The following summer they built a cinder block kitchen and bathroom into the side of a hill, then dragged the garage up on top with horses and a homemade tractor. The steps up & down were six stone steps set into the hill and seven wood steps up to a wood deck the length of the garage, outside. We lived there for a couple years, then almost 2 years in the cellar of a new house, until there was enough money/time to build the rest of the house.
Most people have way more room than they actually need, and most of that is used to house stuff they don't need and seldom use.