Gross internal area GIA

[edit] What are the different ways of measuring the areas of buildings?

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

• Gross external area (GEA).
• Gross internal area (GIA) or gross internal floor area (GIFA).
• 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 planning applications, building regulation applications, property sales, lease negotiations, rating valuations, and so on.

[edit] What is gross internal area?

The UK government's Valuation Office Agency (VOA) Code of measuring practice: definitions for rating purposes suggests that gross internal area (GIA) includes:

• Areas occupied by internal walls (whether structural or not) and partitions.
• Service accommodation such as WCs, showers, and changing rooms.
• Columns, piers, whether free standing or projecting inwards from an external wall, chimney breasts, lift wells, stairwells, and so on.
• Lift rooms, plant rooms, tank rooms, fuel stores, whether or not above roof level.
• Open-sided covered areas (should be stated separately).

Gross internal area excludes:

• Open balconies.
• Open fire escapes.
• Open-sided covered ways.
• Open vehicle parking areas, terraces and so on.
• Minor canopies.
• Any area with a ceiling height of less than 1.5m (except under stairways).
• Any area under the control of service or other external authorities.

The VOA code is in general agreement with the RICS Code of Measuring Practice, other than areas with a headroom of less than 1.5m, which are excluded from the VOA measurement.

The RICS Code of Measuring Practice referred to above is now in its 6th edition, published in May 2015, and remains RICS's current active guidance; a 7th edition is in development, with consultation on the draft closing in October 2026 (RICS has said it does not expect major changes). Since 2018, RICS has also recognised the International Property Measurement Standards (IPMS) - most recently consolidated as a single standard, IPMS: All Buildings, published in January 2023 - as a complementary basis of measurement, particularly for international or cross-border purposes. RICS Property Measurement (2nd edition, 2018), which had incorporated the earlier sector-specific IPMS documents, has itself since been archived and should only be used as a reference. Practitioners should check which basis - the RICS Code of Measuring Practice, IPMS: All Buildings, or, for rating purposes, the VOA Code - is required for the instruction in hand, and record which one has been used.

NRM1: Order of cost estimating and cost planning for capital building work (as amended in October 2022), defines 'gross internal floor area' (GIFA) (or gross internal area (GIA)) as: "the area of a building measured to the internal face of the perimeter walls at each floor level. The rules of measurement of gross internal floor area are defined in the latest edition of the RICS Code of Measuring Practice."

According to 'Elemental Standard Form of Cost Analysis, Principles, Instructions, Elements and Definitions, 4th (NRM) Edition' written by RICS in 2012 and published by BCIS, GIFA includes:

• Areas occupied by internal walls and partitions.
• Columns, piers chimney breasts, stairwells, lift-wells, other internal projections, vertical ducts, and the like.
• Atria and entrance halls with clear height above, measured at base level only.
• Internal open sided balconies, walkways, and the like.
• Structural, raked or stepped floors are treated as a level floor measured horizontally.
• Horizontal floors with permanent access below structural, raked or stepped floors.
• Corridors of a permanent essential nature (e.g. fire corridors, smoke lobbies, etc.).
• Mezzanine areas intended for use with permanent access.
• Lift rooms, plant rooms, fuel stores, tank rooms which are housed in a covered structure of a permanent nature, whether or not above main roof level.
• Service accommodation such as toilets, toilet lobbies, bathrooms, showers, changing rooms, cleaners' rooms and the like.
• Projection rooms.
• Voids over stairwells and lift shafts on upper floors.
• Loading bays.
• Areas with headroom of less than 1.5 m.
• Pavement vaults.
• Garages.
• Conservatories.

It excludes:

• Perimeter wall thickness and external projections.
• External open-sided balconies, covered ways and fire escapes.
• Canopies.
• Voids over or under structural, raked or stepped floors.
• Greenhouses, garden stores, fuel stores and the like in residential property.

And that:

• The GIFA excludes the thickness of perimeter walls, but includes the thickness of all internal walls. Therefore, it is necessary to identify what constitutes a separate building, e.g. the sum of the GIFA of a terrace of buildings, treated as separate buildings, will be different from the terrace treated as a single building.
• Areas of open ground floors and the like should be excluded.
• 'Internal face' means the structural wall or plaster coat applied to the structural wall, not the surface of internal linings installed by the occupier.
• Lift rooms, etc. should be included if housed in a roofed structure having the appearance of permanence (e.g. made of brick or similar building material). Areas covered by enclosures designed solely to mask plant, rooflines, etc. should be excluded.
• The presence of steps or changes in floor levels should be noted.
• Attention is drawn to the exclusion of voids over atria at upper levels and the inclusion of voids over stairs, etc. Where an atrium-like space is formed to create an entrance feature, and this also accommodates a staircase, this does not become a stairwell but remains an atrium measurable at base level only.
• Walkways across an atrium at upper levels should be included in the measurement of upper floors.
• Areas in the roof space intended for use with permanent access should be included in the measurement of upper floors and measured to internal face of the enclosing wall or the roof at floor level.
• Re-entrant balconies, i.e. open sided balconies within the predominant line of the external wall should be treated as open sided balconies and excluded.

[edit] Related articles on Designing Buildings

• Area.
• Cost per m2 of gross internal floor area.
• Floor area ratio.
• Gross development area.
• Gross external area.
• Gross site area.
• International Property Measurement Standards.
• Measurement.
• Minimum space standards.
• Net internal area.
• Site area.
• Space.
• Total useful floor area.
• Treated floor area.

Comments

A question - what about undercroft spaces? Meaning an open area of building that has the same building above it and columns coming down to ground, with entrance to the building at ground.

Good question. Undercroft spaces like this sit in a genuine grey area between the different measurement bases, so the answer depends a bit on which one you're using and what the figure is for.

Under the RICS Code of Measuring Practice (and the VOA's rating code, which follows it), a covered but open-sided space like an undercroft - roofed by the structure above, supported on columns, open to the outside - falls into the "open-sided covered areas" category. These aren't simply added into the headline GIA figure; the guidance is that they should be measured and stated separately, so you'd typically report your building's GIA and then note the undercroft's area alongside it as a distinct figure, rather than folding it silently into one total.

For cost planning purposes under NRM1, the position is more clear cut: open ground floors of this kind are specifically excluded from GIFA/GIA altogether, since they don't carry the same enclosure and fit out as a normal floor.

Either way, the floors of the building sitting above the undercroft are unaffected by any of this - they're enclosed floors like any other and are measured to the internal face of the perimeter walls at that level in the normal way, so they count fully towards GIA.

In practice, the safest approach is to record which basis you're using (RICS Code, NRM1, or IPMS: All Buildings) and state the undercroft area as a separate line item rather than assume it's automatically in or out - that avoids disputes later, particularly for planning submissions or valuations where the distinction matters. If the undercroft is later enclosed (glazed in, gated, etc.) or put to a defined ancillary use such as secure cycle or bin storage, it's worth reassessing it on the specific facts rather than treating the "open" rule as fixed.