IPMS Explained: How IPMS 1, 2 and 3 Compare to GEA, GIA and NIA

A few years ago, almost nobody asked us for IPMS. In the last 12 months, around 30% of our area referencing enquiries have specified it, especially on larger commercial projects. GIA and NIA are still the standard request, though.

In short: IPMS 1 sits closest to GEA, IPMS 2 to GIA, and IPMS 3 (whose internal version is now called IPMS 3.2) to NIA. Close, but not interchangeable.

And whichever basis you use, an area figure is only as good as the evidence behind it.

GEA, GIA and NIA in plain English

IPMS only makes sense next to the three older bases. And the most common confusion I see on area referencing jobs sits right here: not knowing the difference between GEA, GIA and NIA.

• Gross External Area (GEA): the whole building, measured to the outer face of the external walls.
• Gross Internal Area (GIA): measured to the internal face of the perimeter walls, so the external walls drop out.
• Net Internal Area (NIA): the usable space only. It leaves out toilets, plant rooms, stairs, lifts, shared corridors and structural elements such as columns.

Same building, three different answers. So a figure with no label tells you very little.

How IPMS 1, 2 and 3 line up against them

Does each IPMS measure simply rename one of these? No. Each has a near equivalent, matching closely but not exactly. But first, a name change.

IPMS 3 is now IPMS 3.1 and IPMS 3.2

IPMS: All Buildings, published on 12 January 2023, is now the single current IPMS standard. It splits IPMS 3, the measure of space in one occupier’s exclusive use, into two:

• IPMS 3.1: measured externally.
• IPMS 3.2: measured internally, to the internal dominant face (broadly, the main inside surface of each wall; more below) and to the centreline of any wall shared with a neighbour.

IPMS 3.2 is the one that compares with NIA. If a document just says “IPMS 3”, ask which one it means.

All four IPMS measures side by side

IPMS measure	Nearest older basis	Measured to	Key differences	Typical use
IPMS 1	GEA	Outer face of the external walls	Adds External Floor Areas (balconies, roof terraces), stated separately	Planning
IPMS 2	GIA	Internal dominant face	Window reveals (sides of the opening) count where they form that face; adds External Floor Areas	Building costings, reinstatement
IPMS 3.1	No direct equivalent	Outer face of the external walls	An external measure of one occupier’s space	Occupier figures on an external basis
IPMS 3.2	NIA	Internal dominant face and centreline of shared walls	Keeps internal walls, columns and reveals; reports limited-use areas separately	Agency, valuation, property management, facilities management

 

The widest gap, though, is in the pair most people care about.

IPMS 3.2 vs NIA: where the gap is biggest

Office rents are usually quoted on NIA, which is why this pair matters most. Compared with NIA, IPMS 3.2 keeps in:

• internal walls and columns within the occupier’s space
• half of any wall shared with a neighbouring tenant, measured to its centreline
• window reveals where they form the internal dominant face
• the space between free-standing columns and the walls
• limited-use areas, such as space with less than 1.5 m headroom, which UK practice excludes from NIA

So IPMS 3.2 almost always gives a bigger figure than NIA for the same space.

Those limited-use areas, like External Floor Areas, must be reported separately, so you can see exactly what the total contains.

Where a brief is unclear, we also show undercrofts, covered loading bays and mezzanines separately, because quietly including the wrong one can materially overstate the space on offer.

The internal dominant face: where you measure changes the answer

Much of that gap turns on one term: the internal dominant face (IDF). As defined in IPMS: All Buildings, it works like this:

• Take each section of wall in turn and look at its lowest 2.75 m above the structural floor (or up to the ceiling, if that’s lower).
• Whichever surface makes up more than half of that band is the face you measure to.
• If no surface does, or the wall section isn’t vertical, the finished surface counts as the IDF.

That’s straightforward on a flat, plumb wall. In period buildings, though, the walls aren’t uniform: they can bow, sag and lean in the vertical.

So deciding where to take the measurement becomes quite difficult, because the area you come out with differs significantly depending on the height at which you take it.

Take a leaning wall. Because it isn’t vertical, its finished surface counts as the IDF, yet that surface sits in a different place on plan near the floor than at head height.

The standard tells you which face to measure to, not the height at which to draw the line. So the height has to be decided, and it has to be recorded.

Making the figure repeatable and defensible

As surveyors, we need to be able to go back to a property, check those measurements and come back with the same results.

If a client gets another surveyor in to check the calculations, that surveyor needs to understand exactly what we measured and where we measured it to, so they’re comparing apples with apples.

A Disto (a handheld laser measure) and a notepad leave nothing but pen-and-paper figures. Even a total station (an instrument that measures angles and distances) doesn’t capture enough to repeat the survey easily. Both are good tools, but on walls like these they leave a figure that’s very hard to justify.

This is where 3D laser scanning comes into its own:

• The scan produces a 3D point cloud, a measured digital twin of the property.
• We decide, and state, the exact height at which we take a horizontal slice through the point cloud to produce the line for the area calculation.
• We set out accurate survey control (fixed reference points) and use the same datum (the reference level for all heights), so the measurement can be repeated.
• Everything is stored in our QA (quality assurance) system. If a figure is challenged, or the calculation needs taking at a different level, we can go back to that record and see exactly how to vary it.

My test for any area figure is whether it’s accurate, repeatable and defensible. In a non-uniform building, you can’t prove the last two without recording where you measured.

What the gap does to rent

If IPMS 3.2 gives a bigger figure, does the tenant get more space? No. It’s the same space, measured differently. The risk is paying the same rate on a bigger number.

Cushman & Wakefield’s worked example shows how:

• A rent of £300,000 a year on 10,000 sq ft NIA is £30.00 per sq ft.
• Measure the same space at 10,500 sq ft under IPMS, and the same rent works out at £28.57 per sq ft.
• Keep the rate at £30.00 per sq ft on the IPMS area, and the rent becomes £315,000 a year.

The firm found landlords have “no real incentive” to mark rents down when moving to IPMS. So if you’re a tenant, check which area your rent is calculated on.

How big is the gap? I’d expect IPMS 3.2 to come out around 2 to 5% larger than NIA for the same office. That’s an indicative range, not a measured result, and never a conversion factor.

The real difference is building-specific: recessed glazing, curtain walling and lots of columns widen it, and in a period building, where the line was taken changes it too.

And the figure you start from can be a bigger risk than the standard. On one multi-tenanted office block, the client’s areas came from the original design drawings.

Against our as-built measured survey, they were roughly 10% adrift on GIA and slightly further on NIA, because design drawings show what was intended, not what was built, finished and later subdivided.

That error is wider than the gap I’d expect between IPMS 3.2 and NIA. Measure first; debate the standard second.

Which standard applies today, and how to choose

So which standard should you use? It isn’t a contest: IPMS hasn’t replaced the older bases.

The RICS Code of Measuring Practice 6th edition still applies. Typically that means GIA for industrial and warehouse space, and NIA for shops and much office leasing.

If you’ve seen IPMS called “mandatory”, that dates from a 2016 RICS rule for offices. The document that latterly carried it, RICS Property Measurement (2nd edition), was archived on 9 June 2025.

RICS now says members should adopt the appropriate basis from IPMS: All Buildings or the Code, and that “the basis of measurement used should be documented”.

The consultation on a draft 7th edition of the Code closed on 13 October 2026, and RICS says “there will not be major changes”. Check the RICS page for the latest before you instruct.

Choose the basis by what the figures are for

How do you choose between them? Start with the question I ask first on every area referencing job: what will the figures be used for? Estate records, space planning, lettings, valuation, cost planning, acquisition or facilities management?

Then pick the measure that fits that purpose, using the typical uses in the table above as a guide.

Reading or commissioning an area schedule

Whichever basis you choose, the schedule has to show it. We’re often sent areas like “the building is 200 square metres”, with no basis stated at all.

Can you convert a figure like that into IPMS? Not reliably. Conversion only works where the measured data behind the figure, such as wall and column areas, still exists, and inherited figures rarely come with it. If you don’t know the basis, verify it or re-measure before you rely on it for rent, valuation or space planning.

When we issue a schedule through our area referencing service, the basis is clear on its face. Whoever measures for you, expect it to state:

• the property, survey date, issue date and drawing reference
• the exact area definition, with standard and edition (for example, IPMS 3.2 under IPMS: All Buildings, or GIA under the Code 6th edition)
• units and rounding
• what’s been included, excluded or reported separately: mezzanines, plant rooms, cores, stairs, lifts, circulation, common areas, voids, balconies and external areas

Where walls are irregular, also ask for the method, slice level and datum behind the area line to be recorded.

When a client needs both GIA and IPMS 2 for one building, we report them as separate, fully labelled figures.

Never accept a single unqualified “area” total. The bases are measured differently and can’t be used interchangeably, and that applies just as much to NIA and IPMS 3.2.