I do a lot of work early on when it comes to working out how many apartments we can get on a site with zones. We are of course always asked for net and gross areas. We can get the gross external areas easily of course from the zones, but does anyone know how I can get gross internal areas (GIA). All I need is to be able to take off a notional external wall thickness of each zone to give me the gross internal area. I can see you can reduce by a percentage but that just doesnt seem accurate enough for my liking.