If a house's area is given as 150 sq m, what's included in this? Does it comprise the total area of all rooms, including garages and conservatories, or just the ground floor footprint?
Most professionals measure a home's living space on the basis of 'gross internal area' or 'GIA' (sometimes referred to as gross internal floor area). So it's worth sticking to this method where possible.
Put simply, a house's gross internal floor area will include every millimetre of available floorspace, across all the habitable storeys. Or for a more technical definition, GIA is a building's area measured to the internal face of the perimeter walls at each floor level.
On a conventional home, this would include the ground floor and first floor - but it may also stretch to the basement or converted loft space, for example, or indeed any other additional storeys. Conservatories and integral garages would also be included, but external garages (those detached from the main house) and other outbuildings would not be.
Take your example of a 150m2 house. If we were to imagine this as a simple two-storey building with a square or rectangular footprint, that would entail two storeys each offering 75m2 of floorspace. Of course, there are many different potential layout configurations. For instance, it might be that the property's 150m2 of floor area is made up of two rectangular storeys each of 70m2, plus a ground-floor conservatory of 10m2.
So always look to use the gross internal area when you're planning your home, looking through property particulars or using a tool such as Build It's interactive build cost calculator (http://www.self-build.co.uk/build-cost-calculator).
Chris Bates, Deputy Editor, Build It magazine