A worked gross internal area measurement for thresholds

Floorspace thresholds in the English planning system are expressed in gross internal area, and a scheme that measures 980 m² on one basis and 1,030 m² on another sits on either side of the major development threshold. Getting the measurement right is therefore not a drafting exercise. It changes the determination period, the consultation requirements, the report bundle and, on some sites, whether an environmental impact assessment screening is needed at all.

Gross internal area is measured to the internal face of the perimeter walls at each floor level. Everything inside that line counts, with a short list of exclusions. That sentence resolves most disputes, and the rest of this explains the cases where it does not.

Why the measurement basis decides the threshold

The thresholds that bite on floorspace in England include 1,000 m² for major development in the Development Management Procedure Order, 2,500 m² as the point at which a transport assessment is commonly expected, the 2,500 m² default for retail impact assessment in national policy, the residential floorspace figure at which a fire statement is flagged, and the screening thresholds in the Environmental Impact Assessment Regulations 2017.

Each of these is a cliff edge. Fall the wrong side and the application changes character: 13 weeks instead of eight, statutory consultation requirements, a different fee band and, for major development, a different committee route in most schemes of delegation.

Gross external area, measured to the outside face of the walls, is always larger, and on a building with thick walls the difference is substantial. Net internal area, the usable space inside structural walls and excluding stairs, lifts, plant and WCs, is always smaller. Using the wrong one is the most common measurement error in practice, and it usually happens because the figure was lifted from a marketing schedule prepared for a letting.

What is included

Everything within the internal face of the perimeter walls at each floor level, which means:

• Areas occupied by internal walls, partitions, columns, piers and chimney breasts.
• Stairwells, lift wells and escalator wells, measured once at each floor level they serve.
• Service accommodation: WCs, showers, cleaners' cupboards, plant rooms, tank rooms, lift motor rooms and switch rooms.
• Mezzanine floors with permanent access.
• Loading bays that are enclosed, internal balconies and galleries.
• Integral and attached garages.
• Conservatories and enclosed link corridors.
• Areas with reduced headroom, which are included but should be stated separately where headroom falls below 1.5 metres.

The inclusion of plant, circulation and structure is what surprises people coming from a lettings background, where those are precisely the areas stripped out to get to net internal area.

What is excluded, and where disputes arise

The exclusions are shorter:

• The perimeter wall thickness itself and any external projections.
• Open external balconies and open fire escapes.
• Canopies, covered ways and porches that are open to the air on one or more sides.
• Open vehicle parking areas and open yards.
• Voids over stairwells, lift shafts and double-height spaces at the upper floor levels, because the area is counted once at the base.
• Greenhouses, garden stores and similar structures of a temporary or non-permanent nature.

Three disputes recur. Whether a covered but partly enclosed loading area is internal, which turns on how enclosed it actually is rather than on what the drawing labels it. Whether a mezzanine has permanent access, where a fixed stair says yes and a ladder usually says no. And how a double-height atrium is counted, where the correct treatment is to count the floor area once at its base and exclude the void above.

State your treatment of each of these on the drawing. An officer who can see the reasoning will accept it. An officer who finds an unexplained number will recalculate it, and their recalculation will not be in your favour.

A worked measurement on a two-storey unit

This example is illustrative, and the dimensions are chosen to show the method rather than taken from a real building.

A light industrial unit has an external envelope of 24.0 m by 15.0 m, with 300 mm thick perimeter walls. A first-floor office mezzanine with a fixed steel stair runs along one side, 23.4 m by 6.0 m. A canopy 24.0 m by 4.0 m, open on three sides, covers the loading area outside the roller shutter.

1. Ground floor internal dimensions: 24.0 minus 0.6 gives 23.4 m; 15.0 minus 0.6 gives 14.4 m.
2. Ground floor gross internal area: 23.4 multiplied by 14.4 is 336.96 m².
3. Mezzanine: 23.4 multiplied by 6.0 is 140.40 m². It has a fixed stair, so it counts.
4. The void over the warehouse at first floor level is excluded, because that area has already been counted at ground level.
5. The canopy is open on three sides and is excluded.
6. Total gross internal area: 336.96 plus 140.40 is 477.36 m², which rounds to 477 m².

Now the comparison that matters. Measured externally, the ground floor would be 24.0 by 15.0, which is 360.00 m², and the total would read 500.40 m². That is a difference of 23 m² on a small building, arising purely from the measurement basis. Scale the same wall thickness up to a 60 m by 40 m building and the gap between the two bases exceeds 60 m².

At 477 m² the unit sits well below the 1,000 m² major development threshold and below the 2,500 m² point at which a transport assessment is commonly expected. Had the mezzanine covered the full footprint, the total would have been 673.92 m², still below both, but a scheme of three such units on one application would cross 1,000 m² and become major development.

Stating the basis on the drawings

Put the area schedule on the general arrangement drawing, not in a separate document that can be detached from the submission.

It should give the area per floor and the total, the basis used and named as gross internal area, the source of the dimensions, and a note of anything excluded with the reason. Where reduced headroom areas exist, show them separately. Where the scheme is phased or comprises several buildings, give the figures per building and the cumulative total, because the threshold applies to the application as a whole.

Round consistently and say how. A schedule that gives 336.96 in one place and 337 in another invites a query that costs a week.

Where the application is close to a threshold, add a short paragraph in the planning statement setting out the measurement basis and confirming which side of the threshold the scheme falls. Pre-empting the question is faster than answering it later, and it also makes clear which of the 66 report types the scheme does and does not trigger.

Where a different basis applies

Gross internal area is not universal, and using it where something else is required is the mirror image of the error described above.

Community Infrastructure Levy uses gross internal area, but with its own rules on what may be deducted for existing buildings, and those rules turn on lawful use rather than on measurement.

Retail impact is sometimes assessed on net sales area or net tradeable floorspace rather than gross internal area, depending on the wording of the local plan policy. Read the policy rather than assuming.

Residential space standards are expressed as gross internal area, and the nationally described space standard sets minimum internal areas per dwelling and per bedroom, measured on that basis.

Building Regulations and some energy calculations use different conventions again, and a floor area quoted in an energy assessment will not necessarily match the planning figure. Where the two differ in a submission, explain why in the design and access statement rather than leaving a case officer to spot an apparent contradiction.

One honest caveat. Measurement conventions are published and widely used, but they are professional standards rather than statute, and a local plan or a validation checklist can define floorspace differently for a particular policy. Where it does, that definition governs for that policy, and your schedule should show both figures.

This is general guidance, not legal advice. Check the current thresholds and your authority's validation requirements before relying on any figure here. PlanScope provides AI-assisted draft reports for planning applications, and drafts flag missing site facts rather than inventing them. Reports should be reviewed before submission. We do not guarantee planning approval.