The RICS Code of Measuring Practice establishes a hierarchy of definitions designed to facilitate the accurate and consistent measurement, description, and specification of buildings and land. At the foundation of this hierarchy are the three "core definitions"Gross External Area (GEA), Gross Internal Area (GIA), and Net Internal Area (NIA).

By establishing these succinct and precise measurement standards, the Code ensures that property sizes can be uniformly calculated and clearly understood across a wide range of professional purposes, including property valuation, management, town planning, taxation, conveyancing, and the sale or letting of space.

Gross External Area (GEA)

  • Definition: Gross External Area is the area of a building measured externally at each floor level
  • Inclusions:
    • Perimeter wall thickness and external projections
    • Areas occupied by internal walls, partitions, columns, piers, chimney breasts, stairwells, and lift-wells
    • Atria and entrance halls, measured at base level only
    • Internal balconies, mezzanine areas intended for use with permanent access, and structural, raked, or stepped floors
    • Lift rooms, plant rooms, fuel stores, and tank rooms housed in a covered permanent structure
    • Outbuildings sharing at least one wall with the main building, loading bays, and pavement vaults
    • Garages, conservatories, and areas with a headroom of less than 15m
  • Exclusions:
    • External open-sided balconies, covered ways, and fire escapes
    • Canopies, open vehicle parking areas, and roof terraces
    • Voids over or under structural, raked, or stepped floors
    • Greenhouses, garden stores, fuel stores, and the like in residential property
  • Examples/Applications:
    • Town Planning: Used as the basis of measurement for planning applications and approvals, such as site coverage and plot ratio
    • Rating and Council Tax: Used for council tax banding of houses and bungalows, and for the rating of warehouses and industrial buildings in Scotland
    • Building Cost Estimation: The preferred method for calculating building costs of residential property for insurance purposes

Gross Internal Area (GIA)

  • Definition: 
    • Gross Internal Area is the area of a building measured to the internal face of the perimeter walls at each floor level
    •  The "internal face" refers to the brick/block work or plaster coat applied to the wall, not the surface of internal linings installed by an occupier
  • Inclusions:
    • Areas occupied by internal walls, partitions, columns, piers, chimney breasts, stairwells, lift-wells, and vertical ducts
    • Atria and entrance halls, measured at base level only
    • Internal open-sided balconies and walkways
    • Structural, raked, or stepped floors, and horizontal floors with permanent access below them
    • Corridors of a permanent essential nature (eg, fire corridors), and mezzanine floor areas with permanent access
    • Service accommodation (toilets, bathrooms, changing rooms, cleaners' rooms)
    • Lift rooms, plant rooms, projection rooms, and tank rooms housed in a permanent covered structure
    • Voids over stairwells and lift shafts on upper floors
    • Loading bays, pavement vaults, garages, conservatories, and areas with a headroom of less than 15m
  • Exclusions:
    • Perimeter wall thicknesses and external projections
    • External open-sided balconies, covered ways, fire escapes, and canopies
    • Voids over or under structural, raked, or stepped floors
    • Greenhouses, garden stores, fuel stores, and the like in residential properties
  • Examples/Applications:
    • Estate Agency and Valuation: The basis for marketing and valuing industrial buildings, warehouses, department stores, retail warehouses, and food superstores
    • Property Management: Used for calculating service charges for the apportionment of occupiers’ liabilities
    • Building Cost Estimation: A recognised method for calculating building costs

Net Internal Area (NIA)

  • Definition:
    • Net Internal Area is the usable area within a building measured to the internal face of the perimeter walls at each floor level
    •  An area is considered "usable" if it can be used for any sensible purpose connected to the intended use of the premises
  • Inclusions:
    • Atria with clear height above (measured at base level only)
    • Entrance halls, kitchens, notional lift lobbies, and notional fire corridors
    • Built-in units, cupboards, ramps, sloping areas, and steps occupying usable areas
    • Areas occupied by ventilation/heating grilles, skirting, and perimeter trunking
    • Areas occupied by non-structural walls subdividing accommodation in sole occupancy
    • Pavement vaults
  • Exclusions:
    • Parts of entrance halls, atria, landings, balconies, corridors, and circulation areas that are used in common with other occupiers
    • Toilets, toilet lobbies, bathrooms, and cleaners’ rooms
    • Lift rooms, plant rooms, tank rooms, and fuel stores
    • Stairwells, lift-wells, and permanent lift lobbies
    • Internal structural walls, columns, piers, chimney breasts, vertical ducts, and walls separating tenancies
    • Areas under the control of external authorities, such as meter cupboards
    • Space occupied by permanent/continuous air-conditioning, heating, or cooling apparatus that renders the space substantially unusable, or intermittent apparatus protruding 025m or more into the usable area
    • Areas with a headroom of less than 15m
    • Areas rendered substantially unusable because the dimension between opposite faces is less than 025m
    • Vehicle parking areas
  • Examples/Applications:
    • Estate Agency and Valuation: The primary basis of measurement for the valuation and marketing of shops, supermarkets, and offices
    • Rating: The principal basis for the rating of shops, offices, and composite hereditaments
    • Property Management: Used to calculate service charges for the apportionment of occupiers' liabilities

Reference 

  • RICS, Code of measuring practice 6th edition