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