Gross Internal Area (GIA) is the area of a building measured to the internal face of the perimeter walls at each floor level, which includes:
- 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 treated as a level floor measured horizontally;
- Horizontal floors with permanent access below structural, raked or stepped floors;
- Corridors of a permanent essential nature (eg fire corridors, smoke lobbies, etc);
- Areas in the roof space intended for use with permanent access;
- Mezzanine areas intended for use with permanent access;
- Lift rooms, plant rooms, fuel stores, tank rooms which are housed in a covered structure of a permanent nature, whether or not above main roof level;
- 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 1.5m;
- Pavement vaults;
- Garages; and
- Conservatories.
The GIA excludes:
- Perimeter wall thickness 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 property; and
- Open ground floors and the like.
To meet international standards, a slightly different definition can sometimes be applied to offices. However, in this context we are primarily interested with the GIA of the actual building.
Source: RICS