GIA vs NIA: What’s the Difference and Why Your Rent Depends on It

Two documents, same floor, two different numbers. One says 10,000 sq ft. The other says 8,400. Neither is wrong.

Only one of them is the number your rent is charged on.

GIA (Gross Internal Area) is everything inside the building, measured to the internal face of the perimeter walls, at every floor level. NIA (Net Internal Area) is the usable area that survives once structure, stairs, lifts, plant and common parts come out. GIA is always the larger figure. GEA (Gross External Area) is the building measured from the outside, and it is mainly a planning number.

Those definitions take ninety seconds to learn. Working out which one governs your building, and where the figure came from, is the part that costs money.

GIA, NIA and GEA: what each one includes

When people come to us for area referencing, the commonest confusion isn’t GIA against NIA on its own. It’s GIA against NIA against GEA, three legitimate figures for one building, all different. Usually that happens because the person buying the survey has been told to get a price, not to use the data, so nobody has checked which figure the project actually needs.

	What it counts	What it leaves out
GIA	Everything inside the perimeter walls at each floor: internal walls and partitions, columns, stairwells and lift shafts at every level they pass through, WCs, plant rooms, corridors, mezzanines, integral basements and garages	The perimeter walls themselves, open balconies and terraces, external fire escapes, unenclosed canopies and walkways, external bin and meter stores
NIA	The workable floor area inside the demise: offices, meeting rooms, retail selling floor, circulation within the tenancy, kitchens exclusive to the tenant	Everything GIA leaves out, plus structural walls and columns, stairs, lifts and landings, entrance halls and common parts, plant rooms, risers and comms cupboards, meter rooms, WCs outside the demise

Low-headroom space is not treated the same way by both bases. Anything with less than 1.5m of clear headroom is excluded from NIA outright. Under the RICS Code of Measuring Practice it still counts towards GIA, but it should be identified separately on the schedule rather than buried in the total. Under IPMS the same space is measured as a limited use area and stated separately rather than discarded. So sloping roof space and the area tucked under a staircase look like floor. Whether they count as floor depends on which basis you’re quoting.

There is no conversion factor

Tempting shortcut: take the GIA, knock off 15%, call it NIA. It doesn’t work.

There is no fixed ratio between the two, no rule of thumb that holds, and no calculator. What exists is a range. An efficient building might run at 80% to 90% net to gross. Modern open-plan office stock typically sits around 80% to 85%. Older buildings with thick masonry walls and oversized cores can fall to 65% to 70%.

That spread is a quarter of the building, and it depends on how your specific property was built and later carved up.

We proved that to ourselves on a multi-tenanted office block, which I’ll come back to later. When we compared our survey against the client’s existing figures, the GIA variance and the NIA variance came out different sizes. If two areas of the same building drift at different rates, neither can be scaled from the other.

Which basis your property is let on

So which one governs your deal? Market convention is consistent enough to plan around:

• Offices: let, marketed and rent-reviewed on NIA.
• High street retail and supermarkets: NIA.
• Industrial, warehouse and retail warehouse units: GIA.
• New-build and residential marketing plans: GIA.
• Mixed-use buildings: each element stated separately, on its own basis.

If you’re comparing a shortlist, this matters twice over. Two buildings quoted on different bases cannot be compared on £ per sq ft, and the cheaper-looking one may simply have been measured more generously.

What the basis costs you: rent, rates and service charge

Rent is a rate multiplied by an area. Change the basis and you change the bill.

Cushman & Wakefield published a worked example that shows how sharp the effect gets. A headline rent of £300,000 on an NIA of 10,000 sq ft is £30.00 per sq ft. Measure the same space under IPMS and it might return 10,500 sq ft, because IPMS counts columns, the occupier’s share of party walls and sub-1.5m areas that NIA discards. Hold the rate and the rent becomes £315,000 a year. Hold the rent and the quoting rate drops to £28.57. There is no market incentive to mark the rate down.

Your rates bill runs on the same number. The Valuation Office Agency (VOA) measures offices and shops on NIA, and industrial and warehouse property on GIA.

The current rating list in England and Wales took effect on 1 April 2026, based on rental levels at the antecedent valuation date of 1 April 2024. Revaluations now run on a three-year cycle, so the next one is due in 2029. If the floor area the VOA holds for your property started life on a drawing, you pay that error every year of the list.

Service charge follows the same logic: floor-area apportionment is the most common method, and rent-free periods and fit-out contributions are usually calculated off the same figure.

So one wrong number prices your rent, your rates and your share of the running costs, then repeats it every year of a 5, 10 or 15-year term.

Where the figure in your lease actually came from

All of that rests on a number most tenants have never questioned. In my experience, it usually isn’t a measurement of the building as it stands.

Clients arrive with areas taken off design drawings, brochures, historic particulars or an old lease schedule. Design drawings are simply that: what has been designed. They are not necessarily how the building has been built, finished or fitted out.

Back to that multi-tenanted office block. The client’s area calculations had been worked up from the design drawings rather than an accurate measured survey of the property in its current condition.

When we compared the two, the design information was generally quite good and it did marry up well. It was still roughly 10% out on the GIA.

Why?

There wasn’t one standout reason:

• wall thicknesses that differed from the design, because the make-up of the wall as built wasn’t what was drawn
• wall finishes and build-ups adding a little more
• layouts changed from the original design
• further subdivision of office space and additional fit-out in the years since

Some of those have a small impact. Others are more significant. Together they moved a tenth of the building.

Treat any area from a brochure, historic particulars or a lease document as provisional until it has been measured.

Why NIA drifts further than GIA

Now the part that should concern an office tenant. On that same building, the variance on the NIA was larger again than the variance on the GIA.

That follows from what causes the drift. Subdivision and fit-out barely touch the outer envelope, so GIA moves only a little. But every new partition, riser and cupboard eats directly into usable space, and usable space is NIA.

The number your rent is charged on is the more fragile of the two.

We saw the same pattern at a London shopping centre. The client had acquired the centre and needed unit areas so rentable values could be agreed, but the only plans were the original design drawings, which showed one large open space. It had since been subdivided into smaller units. Neither side would proceed without knowing they weren’t being over- or under-charged, so we measured the units as built and produced GIA and NIA calculations, and the leases were negotiated on those.

“Measured to RICS standard” isn’t an answer

That phrase used to settle the question. It no longer tells you enough, because more than one route is live.

RICS Property Measurement (2nd edition) was archived on 9 June 2025 and should now be treated as reference only. The direction is to adopt the appropriate basis from IPMS: All Buildings, effective since 15 January 2023, or the RICS Code of Measuring Practice 6th edition, and to document which basis was used. A 7th edition of the Code is in development but not yet published.

It is worth knowing why the market splits the way it does. While RICS Property Measurement was still live, it required RICS members to report office and residential instructions on IPMS, so landlords, valuers and agents in those asset classes still expect to see an IPMS figure. Industrial, retail and warehouse work, meanwhile, has stayed on the Code of Measuring Practice.

Those routes give different answers for the same space, as the £300,000 example above shows.

IPMS is no longer a fringe request either. Around 30% of our area referencing enquiries over the last twelve months have specified IPMS, particularly on larger commercial projects, although traditional GIA and NIA calculations remain the standard deliverable. Where a client needs both, we issue them as separate labelled figures. We never present a single unqualified “area” total.

What to ask for before you sign, at review, or when the list changes

You don’t need to measure anything yourself. You need to commission properly, and that comes down to a short list of asks.

• The abbreviation, printed on the face of the schedule and the plan. GIA, NIA, GEA or the IPMS basis. We put the appropriate abbreviation against every area calculation we issue, so it is crystal clear what the numbers relate to.
• The standard and edition applied. “RICS” on its own isn’t a standard.
• The survey date, and whether the area was measured or transcribed. A figure lifted off a drawing is not a survey.
• The tolerance worked to. Typically ±5mm at 1:50, ±15mm at 1:100 and ±25mm at 1:200, so another competent surveyor should land within a known margin of your figure.
• How the awkward spaces have been treated. Mezzanines, undercrofts, covered loading bays and low-headroom areas should be shown separately, not quietly absorbed into a total.

Tell whoever you instruct what the figures are for as well: lettings, rating, valuation, space planning and cost planning don’t all need the same basis, and it’s the first question we ask on any area referencing enquiry.

The mistake I see most often is assuming the plans already on file are accurate and current. They rarely are, and the problem usually surfaces as an urgent last-minute survey, at exactly the point when there’s no time left to argue about the answer.

Check the basis before you sign, not after.

Common questions

Does GIA include voids and double-height space?

The void itself isn’t floor area, so it isn’t counted. The floor surrounding it is included at each level where floor exists. A double-height reception therefore adds area once, not twice.

Does my surveyor have to be RICS-registered?

“Surveyor” isn’t a protected title. “Chartered Surveyor” is, and only RICS awards it (MRICS or FRICS). What matters most for lease and rating evidence is that the area schedule is produced to a named RICS standard, using calibrated equipment, by a firm carrying professional indemnity insurance.

Is retail rent just NIA multiplied by a rate?

No. Shop rents and rating assessments are analysed by zoning: 6.1m deep zones from the frontage, with value halving back (Zone B is half Zone A, Zone C a quarter). Zoning is valuation analysis rather than measurement, but the survey has to capture enough dimensions to allow a zoned analysis, so say so in your instruction.