The legal definition of a building footprint is "the outline of a building at all points where it meets the ground". An "original house" means a house as it stood on 1 July 1948 or as it was first built if built after, and the size of permitted extensions are all judged from this. An integral garage within an existing footprint does not limit the use of that space to a garage - it can be re-allocated into general use. A detached new garge would be a completely separate matter, and permission would be judged against whether it would make a material change to the use of the land.