The building footprint would the area that that the building sits on, it will not give you the square meterage of useable space - that will be slight less for a one story building (probably the internal area you mention) i.e. building footprint less and amount for external walls. It will also include a garage if this is attached to the house. If you have a 2 story house it will depend on whether the garage (if any) has rooms above it etc. The measure that would be of most use to you for comparison purposes is the internal square meter figure - this should be the living area of the house.