We can rarely build high enough to allow 3rd floors but I constantly am asked to make the second floor larger than the main and garage combined. More like 30 than 50%. I call them mushroom houses....
Main is entry, a den, maybe a rec room, and a one or two bedroom rental suite, plus garage of course,on the upper is a full 3 or 4 bedroom home....
You end up with a row of posts across the back of the house, yuck. ron