Usual foundations (though it varies in different parts of the USA) would be 8 inches around the perimeter with 6 inches in the "middle" part of the floor. Of c

Usual foundations (though it varies in different parts of the USA) would be 8 inches around the perimeter with 6 inches in the "middle" part of the floor. Of course for commercial or multi-story buildings you would see an increase in the thickness.