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For Business Rate purposes the net internal area (NIA) is the total usable area of each floor within a building, measured to the inside face of the boundary walls (ignoring skirting boards). Areas measured include:

  • kitchens
  • any built-in cupboards or units that occupy usable areas
  • perimeter skirting boards, mouldings and trunking
  • open circulation areas such as atria, corridors and entrance halls
  • Partition walls and other dividing elements

Areas that are excluded are:

  • internal structural walls
  • walls (whether structural or not) enclosing excluded areas
  • piers, columns, chimney breasts, ducts and other projections
  • cleaner cupboards
  • lifts, lift rooms, lift wells
  • stairwells and landings
  • stairwells, entrance halls, atria, landings and balconies used in common or for essential access
  • corridors and other circulation areas used in common with other occupiers or of a permanent essential nature
  • boiler rooms, fuel stores, plant rooms and tank rooms other than those of a trade process nature
  • car parking areas
  • spaces occupied by cooling and heating equipment, air conditioning systems and ducting which renders the space substantially unusable
  • areas with a headroom of less than 1.5m
  • toilets and associated lobbies, unless additional toilets have been installed
  • areas under the control of service or other external authorities

The net internal area (NIA) is used as part of the Business Rates Measuring Method.