Net internal area NIA

[edit] How is the area of a building measured?

The area of a building can be measured in a number of different ways:

• Gross external area (GEA).
• Gross internal area (GIA).
• Net internal area (NIA).
• Total useful floor area (TUFA).
• Usable floor area.

It is very important when describing the area of a building to be clear about which measure is being used, for example in property sales, planning applications, building regulations applications, lease negotiations, rating valuations, and so on.

[edit] What is the net internal area of a building?

The net internal area (NIA) of a building is the usable area measured to the internal finish of the perimeter or party walls, ignoring skirting boards, at each floor level. Net internal area includes all areas that can be used for a particular purpose. The UK government's Valuation Office Agency (VOA) Code of measuring practice: definitions for rating purposes suggests that Net Internal Area includes:

• 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.

Net internal area excludes:

• Internal structural walls.
• Walls (whether structural or not) enclosing excluded areas.
• Piers, columns, chimney breasts, ducts and other projections.
• Cleaner's cupboards.
• Lifts, lift rooms, lift wells.
• Stairwells and landings.
• Stairwells, entrance halls, atria, landings and balconies used in common or for the purpose of 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.

NB: Essential access does not include reception areas or areas capable of use and situated within entrance halls, atria and landings etc.

In relation to the term 'permanent essential nature' the code of measuring practice suggests that, 'apart from areas used in common with other occupiers, corridors excluded from NIA are those of a permanent essential nature, i.e. internal corridors between structural walls (usually found in older buildings). Fire corridors and smoke lobbies which are defined by non-structural walls - but only where they are permanent and essential to any prospective tenant of the property and do not merely serve the needs of the actual occupier. If the latter, they should be included.'

The RICS Code of measuring practice, 6th edition (effective from September 2007 and rolled out globally from May 2015), remains the current RICS standard defining net internal area and is in general agreement with the Valuation Office Agency definition. RICS Property Measurement, 2nd edition, which incorporated the International Property Measurement Standards (IPMS), has since been archived, so UK practitioners measuring NIA for lease, valuation or rating purposes should continue to refer to the Code of measuring practice 6th edition and/or IPMS: All Buildings as appropriate to their instruction. A 7th edition of the Code of measuring practice was out for public consultation at the time of writing, with a closing date of 13 October 2026, so practitioners should watch for its eventual publication.

[edit] Related articles on Designing Buildings

• Area.
• Case notes for rating valuation.
• Floor area ratio.
• Gross development area.
• Gross external area.
• Gross internal area.
• Gross site area.
• International Property Measurement Standards.
• Measurement.
• Measurement of existing buildings.
• Rating valuation.
• Site area.
• Size.
• Space.
• Total useful floor area.

Comments

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Can someone explain the design calculation? from architect point of view.

ie - lets say we have a plot of 25X10 meters in Hendon, London.

How can i calculate what is the allowed Net Ground Floor? and on the Net First Floor..?.

How to decide the allowed building contour based on plot contour?

I read that the building border should be 3 meter smaller than plot border on width and 10 meters on length - so in that case - 15 (length) on 7 (width) - which its net ground floor of 105 SQM. is it right?

what are the rules in terace house plot? in triangle plot?