RICS Code of Measuring Practice: A Guide to GEA, GIA, NIA and IPMS
The RICS Code of Measuring Practice has been the backbone of property measurement in the UK for over four decades. But if you’re trying to work out which standard applies to your building right now, things have got more complicated than they used to be.
For offices and residential properties, the Code’s traditional measurement bases have been replaced by the International Property Measurement Standards (IPMS). But for industrial, retail, and mixed-use buildings, the Code still applies. And with the RICS currently updating the Code during 2025, we’re in something of a transitional period across the industry.
This guide explains what each standard means, how the old maps to the new, and which one you should be using. If you need your building measured to any of these standards, our IPMS Area Assessment and measured building survey services cover them all.
Key takeaways
- The Code (6th edition) still governs industrial, retail, warehouse and mixed-use buildings; offices and residential now use IPMS.
- GEA measures to the outside of external walls, GIA to the inside, and NIA only the usable occupier space.
- IPMS 1 ≈ GEA, IPMS 2 ≈ GIA, IPMS 3.2 ≈ NIA — but the figures don’t match exactly.
- IPMS 3.2 typically produces an area 2–5% larger than NIA, lowering the rate per sq ft for the same rent.
- The Internal Dominant Face is the biggest technical change — and during the transition, always document which basis you’ve used.
In this guide
What Is the RICS Code of Measuring Practice?
The Code of Measuring Practice is an RICS guidance note, now in its 6th edition, that defines how buildings in the UK are measured using three bases: Gross External Area (GEA), Gross Internal Area (GIA), and Net Internal Area (NIA).
First published in 1979, the Code went through six editions. The current 6th edition was published in September 2007 and revised in May 2015 when RICS introduced IPMS for office buildings. At that point, the Code stopped applying to offices. Residential was removed from the Code’s scope in May 2018.
So what’s the position today? The Code still applies to industrial properties, retail units, warehouses, and mixed-use buildings. The RICS Property Measurement 2nd Edition (which incorporated IPMS for offices and residential) was archived in June 2025, and RICS is working on an updated Code.
During this interim, RICS members should use either IPMS: All Buildings or the Code as appropriate, and always document which basis they’ve used.
It’s a messy transitional period. But the underlying principles haven’t changed much.
What Is the Difference Between GEA, GIA, and NIA?
GEA measures to the outside of external walls, GIA measures to the inside of external walls, and NIA measures only the usable floor space, excluding common areas, stairs, lifts, and structural elements.
Those three sentences cover the core idea. But the detail matters, because picking the wrong standard can produce a floor area figure that’s 20–30% different from what someone else expects.
Gross External Area (GEA) is the total footprint of the building measured to the outer face of the external walls at each floor level. It includes wall thicknesses, internal walls, columns, stairwells, lift wells, and plant rooms. You’d use GEA for planning applications, building cost estimates for residential properties, council tax banding, and insurance rebuild calculations.
Gross Internal Area (GIA) is the area measured to the internal face of the perimeter walls. It still includes all the internal stuff (corridors, stairs, lifts, toilets, plant rooms, columns) but strips out the wall thickness. GIA is the standard for valuing industrial and warehouse properties, retail warehouses, department stores, and for estimating construction costs on commercial buildings.
Net Internal Area (NIA) is the usable space that an occupier actually gets. It excludes toilets, plant rooms, stairwells, lift wells, permanent corridors, lobbies, structural walls, and columns. Anything with headroom under 1.5m is also excluded. NIA is what you’ll see quoted when you’re looking at office space to lease, and it’s the basis most retail shop valuations work from.
All three are defined in the RICS Code of Measuring Practice, 6th Edition (PDF).
What Are International Property Measurement Standards?
IPMS is a set of global standards created by a coalition of 88+ professional organisations to give everyone involved in property a single, consistent way of measuring buildings regardless of which country they’re in.
The problem IPMS was built to solve is real. Research by JLL found that the same building could produce area figures varying by up to 24% depending on which country’s measurement standard was used. For international investors and occupiers comparing properties across borders, that’s a serious problem.
The IPMS Coalition published its first standard (for office buildings) in November 2014. RICS made IPMS mandatory for its members measuring offices from 1 January 2016, and extended that to residential buildings from 1 May 2018. In January 2023, the Coalition published IPMS: All Buildings, a single consolidated standard covering every building type. That’s the current version.
IPMS is now required whenever RICS members take on new instructions for office or residential properties. This includes new leases, rent reviews, sales, valuations, and any situation where a physical change to the building triggers re-measurement.
What Do IPMS 1, 2, and 3 Actually Measure?
IPMS 1 measures the full building footprint to the outside of external walls (similar to GEA), IPMS 2 measures to the Internal Dominant Face (similar to GIA), and IPMS 3 measures the exclusive occupier area (similar to NIA).
They sound like direct replacements for the old standards. They’re close, but each pair has specific differences. And those differences can affect the figures you end up with.
IPMS 1 measures to the external face of the external walls and includes any Notional Boundaries, External Floor Areas (balconies, rooftop terraces), and Sheltered Areas. Those external areas are stated separately in the report, so you can strip them out if you need a GEA-equivalent figure. Used for planning, insurance, and construction cost estimates.
IPMS 2 measures to the Internal Dominant Face (IDF) rather than the internal face of the perimeter walls. It can be reported by component areas (labelled A through H), which makes it useful for benchmarking and cost analysis:
- A — walls, columns, and boundaries
- B — vertical penetrations like lift shafts and stairs
- C — technical areas
- D — sanitary
- E — circulation
- F — primary use areas
- G — secondary
- H — other
IPMS 2 is used for costings, reinstatement values, and benchmarking.
IPMS 3 comes in two variants. IPMS 3.1 measures the exclusive occupier area externally (to the outside of external walls and centreline of shared walls). IPMS 3.2 measures internally to the IDF and centreline of demising walls. Standard Facilities (stairs, lifts, toilets, plant rooms) are excluded. IPMS 3.2 is the one that replaces NIA for office lettings and valuations, and it’s the standard you’ll encounter most often in practice.
How Do the Old and New Standards Compare?
IPMS 1 maps roughly to GEA, IPMS 2 to GIA, and IPMS 3.2 to NIA. But each pair has differences that mean the figures won’t match exactly for the same building.
The mapping between IPMS 1 and GEA is the closest. The main difference is that IPMS 1 includes balconies, colonnades, and sheltered areas (canopied loading bays on industrial units, for example) where GEA excludes them. Since IPMS reports these separately, you can subtract them to get very close to a GEA figure.
IPMS 2 versus GIA diverges a bit more. The key difference is measurement to the Internal Dominant Face. In a building with floor-to-ceiling glazing, the IDF might be the inside face of the glass, which sits further in than the internal wall face that GIA measures to. Window reveal areas get included in IPMS 2 but excluded from GIA. Buildings with deep reveals or high glazing ratios show the biggest variance.
The gap between IPMS 3.2 and NIA is the most significant. IPMS 3.2 includes columns and internal structural walls within the occupier’s exclusive area. It includes limited-use areas (like spaces under 1.5m headroom) stated separately. And the IDF measurement adds area in buildings with deep window reveals. The net effect is that IPMS 3.2 typically produces a figure around 2–5% larger than NIA for the same space.
Why this matters for rent: if an office costs £300,000 per year and measures 10,000 sq ft on an NIA basis, that’s £30 per sq ft. The same space might measure 10,500 sq ft under IPMS 3.2, dropping the rate to roughly £28.57 per sq ft. Same building, same rent, different rate. Cushman & Wakefield's analysis of this gap is worth reading if you’re negotiating lease terms.
If you need to understand exactly how your building measures under both old and new standards, our IPMS Area Assessment service provides a full comparison.
What Is the Internal Dominant Face?
The IDF is the inside surface that makes up more than 50% of the lowest 2.75m of each wall section. It determines where you measure to under IPMS 2, 3, and 4.
This is probably the single biggest technical change introduced by IPMS, and it’s caused the most debate among surveyors.
Under the old standards, you always measured to the internal face of the perimeter wall. Simple. Under IPMS, you assess each wall section individually. If glazing covers more than half the surface area in the lowest 2.75m, the inside face of the glass becomes your measurement line. If solid wall dominates, you measure to the wall surface.
Buildings most affected by this are period properties with deep window reveals (where the glass is set well back from the internal wall face) and modern curtain-walled offices with floor-to-ceiling glazing. In both cases, the IDF can sit at a different point than where GIA or NIA would traditionally have been measured.
Getting the IDF right requires careful on-site assessment. It’s one of the reasons 3D laser scanning has become the preferred method for IPMS surveys. A point cloud captures the exact geometry of every wall section, making it straightforward to determine the dominant face with precision.
Which Measurement Standard Should You Use?
For offices and residential properties, IPMS is mandatory for RICS members. For industrial, retail, and mixed-use buildings, the RICS Code of Measuring Practice still applies.
That’s the headline rule. But the detail matters depending on your specific situation.
If you’re taking on a new office lease, negotiating a rent review, buying or selling an office building, or commissioning a valuation of office or residential property, IPMS applies. For international investor portfolios, IPMS is strongly recommended regardless of building type because it enables cross-border comparison.
For warehouse valuations, industrial properties, and retail units, the Code of Measuring Practice 6th Edition is still the standard. GIA for industrial and warehouses. NIA for retail shops.
Business rates are a separate matter entirely. The Valuation Office Agency (VOA) uses its own measurement code, which typically applies NIA for offices and GIA for industrial properties. This doesn’t always align perfectly with either IPMS or the RICS Code.
And if you’ve got an existing lease that references NIA or GIA, there’s no automatic requirement to re-measure under IPMS unless a triggering event occurs (a new lease, physical alteration, rent review, or similar).
During the current transitional period, the most important thing is documentation. Whatever standard you use, record it clearly in your report or lease schedule. Don’t assume everyone knows which basis a floor area figure is based on.
Frequently Asked Questions
Yes, but only for certain property types. The Code’s 6th edition still applies to industrial, retail, warehouse, and mixed-use buildings. For offices (since January 2016) and residential (since May 2018), IPMS has replaced the Code’s measurement bases. RICS is currently updating the Code during 2025, and the RICS Property Measurement 2nd Edition was archived in June 2025. During this transitional period, RICS members should use IPMS: All Buildings or the Code as appropriate and document which basis they’ve used.
GIA (Gross Internal Area) measures the full internal area to the inside face of the perimeter walls, including corridors, stairs, lifts, toilets, and plant rooms. NIA (Net Internal Area) measures only the usable occupier space, excluding all those common and structural elements. GIA is always the larger figure. Under IPMS, the approximate equivalents are IPMS 2 (similar to GIA) and IPMS 3.2 (similar to NIA), though the figures won’t be identical due to differences in measurement points and inclusions.
Almost always, yes. IPMS 3.2 includes columns, internal structural walls within the occupier’s demise, and limited-use areas (stated separately), all of which NIA excludes. Measurement to the IDF rather than the internal wall face can also add area in buildings with deep window reveals. The increase is typically 2–5% but varies by building. A larger measured area with the same total rent should produce a lower rate per square foot.
IPMS is mandatory for any new instruction involving office buildings (since 1 January 2016) or residential buildings (since 1 May 2018). This covers new leases, rent reviews, sales and purchases, revaluations, and re-measurement following physical changes. Industrial, retail, and mixed-use buildings still use the RICS Code of Measuring Practice. Clients can request an alternative standard in writing, but the surveyor must document the departure.
Because IPMS 3.2 typically produces a larger floor area than NIA, the rent per square foot should theoretically decrease to keep the total rent the same. A 10,000 sq ft NIA office at £30/sq ft (£300,000 total rent) might measure 10,500 sq ft under IPMS 3.2, making the equivalent rate around £28.57/sq ft. In practice, landlords don’t always reduce rates proportionally. Tenants should always check which measurement basis is being used and compare properties on a like-for-like basis.
The IDF is the inside surface that covers more than 50% of the lowest 2.75m of each wall section. If glass dominates that area, you measure to the inside face of the glazing. If solid wall dominates, you measure to the wall surface. This is the most significant technical change introduced by IPMS and replaces the old approach of simply measuring to the internal face of the perimeter wall.
Get Your Building Measured to the Right Standard
Whether you need an IPMS area assessment, a measured building survey with floor areas to any standard, or simply want to talk through which measurement basis applies to your property, we’re happy to help.
As an RICS-regulated firm with in-house 3D laser scanning and chartered surveyors experienced in both IPMS and the Code of Measuring Practice, we can measure your building accurately and advise on the correct standard for your situation.