Living space in Paris is tres expensive! A working architect managed to get a tiny apartment for two, only 24 square meters (258 sf). Installing a loft made it 31 square meters (344 sf), which is still tiny. But he's an architect; let's see what he can do with it.
Putting the bathroom and closet below the loft bedroom was pretty smart, considering the place didn't have a bathroom at all before! You might never need such extreme space-saving ideas, but there are design principles here that anyone can adapt.