Adding nother floor

I am trying to add another floor above the ground floor. The upper floor is not the same size (does not shaeer the same footprint) as the ground floor. I cannot delete the sections of wall and floor that are not needed for the upper floor. As an example if the ground floor is 10-metres x 5-metres and the upper floor is 5-metres x 5-metres I cannot delete the wall sections and floor area from the upper floor that are not required. Can anyone advise on how I may be able to do this