Gross Internal Area Definitions

GIA is the area of a building measured to the internal face of the perimeter walls at each floor level. Including:

  • Areas occupied by internal walls and partitions
  • Columns, piers, chimney breasts, stairwells, lift-wells, other internal projections, vertical ducts, and the like
  • Atria and entrance halls, with clear height above, measured at base level only
  • Internal open-sided balconies, walkways, and the like
  • Structural, raked or stepped floors are property to be treated as a level floor measured horizontally
  • Horizontal floors, with permanent access, below structural, raked or stepped floors
  • Corridors of a permanent essential nature (e.g. fire corridors, smoke lobbies)
  • Mezzanine floor areas with permanent access
  • Lift rooms, plant rooms, fuel stores, tank rooms which are housed in a covered structure of a permanent
  • Nature, whether above the main roof level or not
  • Service accommodation such as toilets, toilet lobbies, bathrooms, showers, changing rooms, cleaners' rooms, and the like
  • Projection rooms
  • Voids over stairwells and lift shafts on upper floors
  • Loading bays
  • Areas with a headroom of less than 1.5 m
  • Payment vaults
  • Garages
  • Conservatories


  • Perimeter wall thicknesses and external projections
  • External open-sided balconies, covered ways and fire escapes
  • Canopies
  • Voids over or under structural, raked, or stepped floors
  • Greenhouses, garden stores, fuel stores, and the like in residential

Last Updated on Wednesday, May 8, 2024