Ask three people for the floor area of the same building and you can get three different figures, all of them correct.
That isn’t sloppiness. It’s because “the area” of a building doesn’t exist as a single number. Gross internal area is one basis among several, and even GIA has two published definitions in England and Wales that contradict each other.
So here’s what GIA is, what the rules put in and leave out, where they diverge, and how to tell whether the figure sitting on your desk can actually be used for what you need it for.
Start with what the figure is for, not the tape measure
When an area referencing enquiry reaches us, my first question isn’t about the building. It’s what the figures will be used for: estate records, space planning, lettings, valuation, cost planning, acquisition or facilities management.
That answer decides the basis. Only then do we agree whether you need whole-building, floor-by-floor, room, departmental, demised or common-area figures, and how any unusual spaces will be treated, all before a quote goes out.
It matters because these figures carry money: community infrastructure levy bills, rateable values, service charge apportionment, rents, cost plans, and for residential conversions, whether the scheme is lawful at all.
Which leads to the most expensive mistake I see: inheriting a figure nobody has verified.
On a multi-tenanted office block, the client’s areas had been calculated from the original design drawings rather than an as-built measured survey. Our survey came out roughly 10% adrift on the GIA, with a slightly larger variance again on the NIA.
There was no single culprit: thicker walls than designed, different wall build-ups and finishes, changed layouts, later subdivision and fit-out.
Design drawings only show what was intended, not how the building was ended up.
That gap can stall a deal.
A client who had bought a London shopping centre couldn’t agree leases on vacant units because the only plans were outdated design drawings showing one open space where years of subdivision had created several smaller units. An as-built survey with GIA and NIA calculations things back on track. That’s the value of getting accurate, recent drawings from a reputable firm.
Purpose first, basis second, measurement third.
Only ever trust GIA when it’s from drawings, particulars or a lease, where those drawings have been verified as accurate and current.
What gross internal area actually is
GIA is the area of a building measured to the internal face of the perimeter walls at each floor level, with every floor added together. That’s the whole definition.
It comes from the RICS Code of Measuring Practice, 6th edition, effective 1 May 2015, which is still the live code. A 7th edition is out for consultation until 13 October 2026, with no major changes expected.
One caveat on scope. The Code is not the whole story for offices and residential buildings. Under the RICS Property Measurement professional statement, RICS members have had to measure and report offices on the IPMS basis since 1 January 2016, and residential buildings since the 2nd edition took effect on 1 May 2018.
GIA under the Code still does the work for cost planning, rating on GIA-valued classes, planning and the rest of the uses the Code lists. But on an office or a flat, expect an IPMS figure alongside it.
Seen GIFA on a cost plan? Same measurement: gross internal floor area.
Two parts of that definition cause most of the trouble. “Internal face of the perimeter walls” means the thickness of the external walls is out, while everything inside them is in, including internal walls and partitions.
And “each floor level” means every floor, above and below ground. Basements count. Pavement vaults count. Roof space with permanent access counts, measured to the internal face of the enclosing wall or roof at floor level.
What GIA includes, and what it leaves out
These two lists are what most people came for, and they are shorter than you would expect.
Included in GIA:
- Internal walls and partitions, structural or not
- Columns, piers, chimney breasts, other internal projections and vertical ducts
- Stairwells and lift wells
- Service accommodation: WCs, toilet lobbies, bathrooms, showers, changing rooms and cleaners’ rooms
- Basements and pavement vaults
- Roof space and mezzanines with permanent access
- Conservatories
Excluded from GIA:
- The thickness of the perimeter walls and any external projections
- External open-sided balconies, covered ways and fire escapes
- Canopies
- Open vehicle parking areas and roof terraces
- Greenhouses, garden stores, fuel stores and the like at residential properties
Then there is the rule people get wrong most often. It only makes sense when you see both halves side by side.
An atrium or entrance hall with clear height above is measured at base level only, so the void above it drops out on the upper floors. But the void over a stairwell or lift shaft is included on every floor it passes through.
An entrance space that happens to contain a staircase is still an atrium, so it takes the atrium treatment.
The 1.5m headroom rule: in for CIL, out for rating
Here is where two published standards openly disagree.
Under the RICS Code, areas with headroom below 1.5m are included in GIA. Under the VOA code of measuring practice for rating purposes, which applies in England and Wales, they are excluded, except under stairways.
- For CIL, those areas are in. The CIL Regulations don’t define GIA, and the appointed persons who decide CIL appeals treat the RICS Code definition as the generally accepted method of calculation. CIL is charged per square metre with indexation, so an area error is a cash error.
- For business rates on GIA-valued classes, they are out. New rateable values take effect on 1 April 2026, based on rental values as at 1 April 2024. If you are checking a 2026 assessment, you need GIA on the VOA basis, not the RICS one, and not the figure written into your lease.
Same building, same surveyor, two correct GIA figures. Which is exactly why an unlabelled area total is worth so little.
GIA is not GEA, NIA or IPMS 2
The most common confusion we are asked to untangle is the difference between GIA, NIA and GEA. It is rarely the client’s fault: the person buying the survey has usually been told to get costs, not to understand the deliverable.
Plenty of figures reach us with no label at all. “The building is 200 square metres” tells me almost nothing.
- GEA is measured externally at each floor level and includes the thickness of the perimeter walls. If someone asks you for the “gross area”, ask which one they mean. The difference is the walls.
- NIA is the usable, lettable area: GIA less internal structural walls, stairwells and lift wells, plant and machinery rooms, common WCs and cleaners’ rooms, and landlord common areas.
- IPMS 2 is measured to the Internal Dominant Face, the inside surface making up more than 50% of the lowest 2.75m of each wall section measured from the structural floor, and it excludes voids over stairs and atria above the lowest level (IPMS: All Buildings, January 2023). That is the opposite of the RICS Code’s treatment of stairwell voids.
Traditional GIA and NIA are still the standard request, but IPMS has grown quickly. In the last 12 months, around 30% of our area referencing enquiries have specified it, mostly on larger commercial projects.
It is landlords, investors, agents, valuers, property managers and larger occupiers doing the asking, because they need consistent area data across a portfolio or across international markets.
Where a client needs both, we report them as separate, fully labelled figures, each with its standard, edition, survey date and drawing reference. We never present a single unqualified “area” total, because GIA and IPMS 2 are measured differently and shouldn’t be used interchangeably.
Which basis for which job
The Code’s applications table maps purpose to basis. The ones that come up most:
- Cost estimation for non-residential building: GIA
- Rating in England and Wales: GIA for industrial premises, warehouses, retail warehouses, department and variety stores and food superstores; NIA for offices and shops
- Agency and valuation: NIA for offices, NIA or retail area for shops, GIA for superstores and retail warehouses, GIA for valuing new homes for development
- Service charge apportionment: GIA, sometimes NIA
- Town planning site coverage and plot ratio, and residential insurance: GEA
- Council tax: GEA for houses and bungalows, effective floor area for flats and maisonettes
One more that catches residential developers. Since 6 April 2021, permitted development rights grant no permission for a dwellinghouse under 37m² GIA, across Class MA, M, O and the other residential classes. On a conversion, GIA isn’t a marketing number. It’s a lawfulness test. The March 2024 relaxation of Class MA dropped the old 1,500 m² floorspace cap and the three-month vacancy condition (SI 2024/141), but the 37m² floor area test stayed exactly where it was.
How GIA is measured, floor by floor
The measuring principle takes four lines to state:
- Measure inside face to inside face across the floor, in both directions.
- Where an internal wall interrupts the run, measure to it, note its thickness, carry on past it, then add those internal wall thicknesses back in.
- Break irregular footprints into rectangles and total them.
- Repeat for every floor, above and below ground, then sum the floors.
That is here so the rules make sense in practice, and it is fine for informal use: sizing a room, sanity-checking an agent’s particulars. It is not fine once the figure feeds CIL, a rateable value, a lease, a valuation or a consent, because the exposure stops being proportionate to the saving.
If you are commissioning a survey, name the basis in the specification. RICS Measured surveys of land, buildings and utilities, 3rd edition gives the measured building survey specification tick-boxes for GEA, GIA, NIA and IPMS 1, 2 and 3. If you don’t tick it, it isn’t a deliverable.
Accuracy is a specification decision too, and it follows the purpose. The Code’s own illustration is a good one: pacing out a car park to ±10% may be perfectly adequate, while better than ±1% may be expected for the NIA of City of London offices.
The spaces that decide the number
In twenty-odd years of measuring buildings across England and Wales, almost every disputed area figure I have seen turned on a handful of awkward spaces rather than on the arithmetic. Undercrofts. Covered loading bays. Low-headroom space under a sloping roof. Mezzanines.
When a building has them, we don’t make assumptions. We confirm the intended use of the figures, agree the basis, and record clearly how each space has been treated, cross-referenced to the drawings.
Where the brief is unclear, we show those areas separately rather than quietly including or excluding them. A mezzanine may sit inside a gross internal area, but it needs to be identified. An open undercroft or covered loading area may fall outside the main internal total, depending on the agreed standard and purpose.
Get that call wrong and the consequences are commercial, not academic. Fold a 150 m² mezzanine or covered loading area into a reported lettable or usable total when it should have been excluded, and you have materially overstated the space available to a tenant, a buyer or a valuer.
Showing areas separately isn’t just a courtesy, either.
Open-sided covered areas, low-headroom areas, car parking and mezzanines are all to be stated separately under both the RICS and VOA codes.
What a properly labelled area schedule says
The deliverable is the schedule, not the number. Hold any figure you are handed against this.
On the face of the document:
- The property, the survey date and the issue date
- The drawing reference
- Units and rounding
- The exact area definition: GIA, NIA, GEA or the relevant IPMS basis
- The measurement standard and the edition used
Then an explicit statement of what has been included, excluded or reported separately: floors, mezzanines, plant rooms, cores, stairs, lifts, circulation, common areas, voids, balconies and external areas.
That way you understand exactly what the figures represent and can compare them properly with any historic area records. It also means the figure survives being reused: a labelled RICS 6th edition GIA can be reconciled against a rating basis or an IPMS schedule, whereas “200 square metres” can only be measured again.
If your schedule doesn’t say which standard and edition it was measured to, you don’t have an area. You have a number.
Need an area schedule you can rely on?
Tell us what the figures are for and we will confirm the right basis, measure it, and label it properly. Free, no-obligation quote.
