Net Internal Area (NIA) is broadly the usable area within a building measured to the face of the internal finish of perimeter or party walls ignoring skirting boards and taking each floor into account.
NIA will include:
- Perimeter skirting, moulding, or trunking
- Any built in units or cupboards occupying useable areas (subject to height exclusion below)
- Partition walls or similar dividing elements
- Open circulation areas and entrance halls, corridors and atria (but see 9 and 10 below)
NIA will exclude:
- Toilets and associated lobbies
- Cleaners’ cupboards
- Lift rooms, boiler rooms, tank rooms, fuel stores and plant rooms other than those of a trade process nature
- Stairwells, lift wells, those parts of entrance halls, atria, landings and balconies used in common or for the purpose of essential access
- Corridors and other circulation areas where used in common with other occupiers or of a permanent essential nature
- Areas under the control of service or other external authorities
- Internal structural walls, walls (whether structural or not) enclosing excluded areas, columns, piers, chimney breasts, other projections, vertical ducts etc
- The space occupied by permanent air conditioning, heating or cooling apparatus and ducting which renders the space substantially unusable having regard to the purpose for which it is intended
- Areas with headroom of less than 1.5m
- Car parking areas
Source: Valuation Office Agency
Therefore, the usable area within a building measured to the face of the internal walls should be provided.
Whilst this is not expected to be accurate to the nearest 1m2, the closest estimate possible for the NFA should be entered. This is to allow for any possible subsequent adjustment to the size of the development.