How floorspace is measured for a major development test

Floorspace measurement for a planning threshold is one of those questions where the statutory wording is thinner than the consequences. The Development Management Procedure Order 2015 makes development major where the floor space to be created is 1,000 square metres or more, or where the site is one hectare or more. It does not define which measurement convention applies to that floor space, and the fee regulations use their own basis. So two competent people can measure the same building and reach figures that straddle the threshold. The defence is to state the basis you used, show the calculation and let the officer check it.

Which measure the threshold uses

Start with the words. The major development test refers to floor space to be created by the development, which does two things at once. It excludes existing floorspace that is simply being reused, and it refers to creation rather than net change.

That is why a change of use with no building work does not reach the floorspace limb at all. No floor space is created, so the question becomes whether the site is one hectare or more, and on most town centre change of use schemes it is not. The scheme is a minor application however large the building is.

Fees are a separate regime with a separate instruction. The fee regulations set out how floorspace is to be worked out for fee purposes, and that convention does not automatically govern the major development test. Treat the two calculations as two pieces of work with the same inputs.

Gross internal area and what it includes

Gross internal area is the most widely used basis in validation checklists, local plan policies and viability work, and the recognised definitions sit in the measurement standards published by the professional institutions.

It is measured to the internal face of the perimeter walls, and it includes internal walls and partitions, columns, stairwells, lift wells, plant rooms and service risers, toilets, and the area beneath a mezzanine. It includes areas with a reduced headroom, with the standards treating very low headroom separately.

It excludes external open balconies, open fire escapes, canopies and the thickness of the perimeter walls themselves.

The practical effect in a residential block is that gross internal area is substantially larger than the sum of the flats, because cores, corridors, bin stores and plant are all inside it. A schedule that gives only the net saleable area is not answering the threshold question.

Where gross external area appears instead

Gross external area is measured to the outer face of the perimeter walls and includes the wall thickness, so it is always the larger figure.

It turns up in two places. Planning application fee calculations, where the regulations direct an external measurement, and some older local plan policies and supplementary documents drafted before measurement standards were widely adopted.

On a building with 300 millimetre external walls, the difference between gross external and gross internal area over several storeys is easily tens of square metres. Near a threshold that difference is the whole argument, which is why naming the basis on the drawing matters more than the number itself.

Mezzanines, plant and covered external space

Three recurring problems.

Mezzanines count as floorspace, and inserting one can itself be development. Section 55(2A) of the Town and Country Planning Act 1990 makes the insertion of a mezzanine floor into a building used for retail purposes development where the gross floor space of the mezzanine exceeds 200 square metres. Below that, no permission is needed for the insertion, but the resulting floorspace still exists for any later calculation.

Plant rooms, substations and service risers are inside gross internal area even though nobody occupies them. Rooftop plant enclosed within a structure counts. Open rooftop plant on an uncovered deck generally does not.

Covered but open-sided areas are the grey zone. A canopy over a loading bay, a covered service yard or an undercroft with open sides is usually excluded from gross internal area and may be included in gross external area. State how you treated it, because an officer may disagree and you want the disagreement to be visible rather than buried.

Change of use where nothing is built

A change of use application that creates no new floorspace cannot cross the 1,000 square metre limb, since nothing is created.

Three consequences follow. Site area becomes the only route to major status, so measure the application site boundary carefully, including any land needed for access, parking and servicing. The fee is calculated under the change of use provisions rather than on floorspace. And thresholds in local policies that use the words existing floorspace, rather than floorspace created, may still bite, so read each policy wording rather than applying one rule everywhere.

The same care applies to the referral thresholds that operate in London, covered in Greater London Authority referable thresholds, which use their own definitions.

Net additional floorspace after demolition

Where existing buildings are demolished, three different figures exist for the same scheme and all three get used.

Floor space created, which is the gross figure for the new buildings and the one the major development test refers to. Net additional floorspace, which is the new figure minus the demolished figure, used in many local policies and in transport and viability work. And chargeable area for the Community Infrastructure Levy, which deducts existing buildings only where they satisfy the regulations on lawful use for a continuous period before the scheme.

Mixing these up produces a scheme that is major on one measure and minor on another, and an officer who concludes you are manipulating the figures. Set out all three in a short schedule and say which policy each one answers.

Recording the measurement so it can be checked

Produce a floorspace schedule as a document in its own right, not as a note on a drawing.

List each floor of each building separately, give the area to one decimal place, state the basis, and name the measurement standard used. Mark the measurement line on the plans. List every item that was included or excluded as a judgement, with one line of reasoning each. Give the site area with the application boundary shown and the area stated in both square metres and hectares.

Where the figure sits within a few per cent of a threshold, say so openly and explain which side you consider it falls. Officers deal with borderline cases constantly and respond far better to a stated position than to a number that quietly lands at 998 square metres. The same transparency helps where the scheme sits on the line between a single dwelling and something larger, discussed in the householder or minor single dwelling line.

The schedule feeds straight into the documents that use it: the transport assessment trip generation, any environmental impact assessment screening request and the viability inputs. Keep one source figure and reference it everywhere.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

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