Last edited 30 Jan 2019

Net internal area NIA

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

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.

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 covers all of those 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:

Net internal area excludes:

It suggests that, 'essential access' '...will 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' it 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.'

RICS Guidance Note, A guide for Property Professionals, 6th Edition Code of measuring practice 2007, is in general agreement with the Valuation Office Agency definition, of net internal area.

[edit] Related articles on Designing Buildings Wiki


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?

Designing Buildings Anywhere

Get the Firefox add-on to access 20,000 definitions direct from any website

Find out more Accept cookies and
don't show me this again