Gross Internal Area, or GIA, is the area of a building measured to the internal face of the perimeter walls at each floor level. It is measured according to the Royal Institute of Chartered Surveyors (RICS) Code of Measuring Practice.
What is included?
- 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
What is excluded?
- 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, January 28, 2026