suspended slab = knock over & start again

more to it than just the footing size, unless verticore bricks were used in the single story construction it will be a nightmare, most single story of that vintage used horizontally cored brickwork for internal walls rendering them useless as loadbearing elements

light weight stud work = do-able but cheap, house will always be considered as a 'loft conversion',insulating, heating & cooling of said addition will always be costly