Floorspace measurement and which figure a threshold uses

The gross internal area planning threshold that makes a scheme major development, the floorspace figure on the application form, the area the fee is calculated from and the area the Community Infrastructure Levy is charged on are not always the same number. They are produced by different conventions, from the same drawings, and a scheme can sit above one threshold and below another by a few square metres.

Gross internal area is the one that does most of the work. It is measured to the internal face of the perimeter walls at each floor level, and it includes internal walls and partitions, columns, stairwells, lift shafts, plant rooms, corridors and internal circulation. It excludes external balconies, open external walkways and, in most conventions, the thickness of the external walls themselves.

The measurement conventions in use

Four conventions turn up in planning work, and mixing them is the source of most of the arithmetic disputes.

The International Property Measurement Standards, adopted through RICS professional guidance, are gradually replacing the older terms, with IPMS 1 broadly corresponding to gross external area and IPMS 2 to gross internal area. Planning documents still use the older labels, so name the standard you used and its edition rather than assuming the reader shares your convention.

Between gross external and gross internal on a masonry building, the difference is roughly the perimeter multiplied by the wall thickness at each floor. On a small footprint with thick walls that is a material percentage, which is exactly the situation in which threshold arguments arise.

Which figure the application form asks for

The standard application form asks for existing and proposed gross internal floorspace, in square metres, and for the number of proposed non-residential floorspace changes by use. It asks for the figure to be given by use class where a scheme is mixed.

Two practical points. Give the existing figure for what is lawfully there now, not for what the last approved drawing showed, because unimplemented consents do not change the baseline. And where a building is being partly demolished, state the demolished area separately rather than netting it off silently, because the officer needs both figures for different purposes.

If a scheme includes a mezzanine, it counts. If it includes a void over a double-height space, that void is not floor area at the upper level. Both of those are frequent correction requests.

Floorspace for the major development test

Article 2 of the Town and Country Planning (Development Management Procedure) (England) Order 2015 makes development major where the floor space to be created is 1,000 square metres or more, or where the site area is 1 hectare or more. The Order does not define floor space, and the working convention across authorities is gross internal area.

"To be created" is doing something specific. It is the new floorspace, not the total after completion, so a 600 m² extension to a 2,000 m² building creates 600 m² and is not major on the floorspace limb. Demolition and rebuild is harder, and authorities differ on whether a like-for-like replacement creates the full new area or only the net increase. Ask, and put the answer in writing.

Major status matters for the determination period, the publicity requirements, and which statutory consultees are engaged. Where the scheme is close to 1,000 m², resolving it early avoids a validation dispute that takes more time than the argument is worth. Our note on statutory consultees that appear as a scheme grows covers what changes once you are over the line.

Floorspace for fee calculation

Planning fees for buildings other than dwellings are banded by the gross floor space to be created, under the fees regulations as amended. The measurement basis for the fee is set out in the regulations and the accompanying guidance rather than in any professional standard, so read the guidance note published with the current fee schedule rather than applying gross internal area by habit.

Fees are indexed annually, with the uplift taking effect at the start of the financial year, so check the schedule in force on the day of submission. An underpaid fee means the application is not valid, and the determination period does not start until it is.

Where a scheme mixes dwellings and other floorspace, the fee is usually calculated for each element and the higher or the aggregate applies depending on the category. The local authority's validation team will confirm, and a short email exchange costs less than a returned application.

Floorspace for levy liability

The Community Infrastructure Levy uses gross internal area, and it uses it consistently, which makes it the most predictable of the four calculations. The chargeable amount is based on the net chargeable area: new gross internal area less the gross internal area of any qualifying existing buildings that are being demolished or changed in use.

The in-use building test controls the deduction. A building qualifies where it has been in lawful use for a continuous period of at least six months within the three years ending on the day planning permission first permits development, and the evidence for that is documentary rather than architectural.

Mezzanines count towards gross internal area for levy purposes. So does plant space, and so does an undercroft car park where it is enclosed. That last one catches people, because a car park contributes nothing to value and a great deal to area.

Common measurement errors

The same handful of mistakes recur across submissions.

• Measuring to the internal face of internal partitions, which produces something closer to net internal area and understates gross internal area
• Omitting stair and lift enclosures at each floor level, when gross internal area counts them at every level
• Including external balconies and open walkways
• Forgetting the plant room, the tank room and the bin store
• Netting demolished floorspace off the created figure before the officer has seen both
• Applying a residential space standard measurement, which uses a headroom rule, to a gross internal area schedule
• Rounding each room and then summing, rather than measuring the enclosed envelope

Where ceilings slope, the residential space standard convention counts floor area with a minimum headroom, and that is a different measurement again. Keep the space standard schedule separate from the gross internal area schedule, and label both. Our note on housing mix and space standards as schemes grow sets out where the space standard figures bite.

Recording the basis on the drawings

Put an area schedule on the drawings, not in a covering letter. It should show, per floor and per use: gross internal area, the convention and standard applied with its edition, the date and revision of the drawing the areas were taken from, and a total. Add the existing areas on the same basis, and a separate line for anything to be demolished.

Then state the derived figures in the covering document: the floorspace created for the major development test, the gross floor space for the fee, the net chargeable area for the levy, and any threshold figures relevant to specialist reports. Where a scheme is near the 2,500 m² gross floorspace default threshold for retail impact, a Retail Impact Assessment may be needed unless the local plan sets a different figure, and highway authorities set their own floorspace triggers for a Transport Assessment. Screening for an Environmental Impact Assessment works on site area and development area rather than internal floorspace, and where a full assessment is required the Environmental Statement must be prepared by competent experts under the 2017 Regulations.

One discipline saves most of the trouble. Fix the area schedule, issue it as a controlled document, and require every consultant to quote figures from it rather than remeasuring. Energy, carbon and levy calculations all depend on it, as our note on energy and carbon evidence as a residential scheme scales shows, and a discrepancy between two consultants' areas is one of the easiest things for an objector to find.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.