Measuring gross internal area for a levy calculation

The gross internal area CIL calculation uses a measurement convention that differs from the one architects use for sales areas, and the gap between the two is where unexpected levy bills come from. Internal circulation counts. Plant rooms count. A basement car park inside the building envelope counts. None of those appear in a net sales area schedule, and a levy figure built from the wrong schedule will be corrected by the collecting authority, upwards.

Measure it once, properly, and carry the same figures through the forms, the appraisal and the drawings.

Which measurement convention applies

The Community Infrastructure Levy Regulations 2010 calculate liability by reference to gross internal area without setting out a full definition, so collecting authorities apply the established surveying definition of gross internal area.

That means measuring to the internal face of the external walls, at each floor level, including everything inside that line. It is not the external footprint, it is not the net internal area, and it is not the net sales area. Three different figures for the same building, and only one of them is the chargeable one.

State on your schedule which convention you have used and at which floor levels you measured. Where a dispute arises later, a schedule that names the convention is defensible and one that just gives a number is not.

Spaces counted and spaces excluded

The division is consistent once you accept that the measurement is about enclosed volume rather than usable space.

Internal car parking is the one that produces the largest surprises. A residential scheme with undercroft parking inside the building line is carrying chargeable floorspace that generates no revenue, and the levy on it has to be budgeted for at land appraisal stage rather than discovered at liability notice stage.

Low headroom areas are worth measuring separately rather than ignoring. Record them, state the headroom, and exclude them openly, so the authority can see you considered them.

Mezzanines, voids and plant rooms

Three recurring arguments.

A mezzanine with permanent access counts as floorspace at its own level. Permanent access means a fixed stair or lift, not a ladder, and a mezzanine reached only by a loose ladder is usually excluded. Where a scheme includes a mezzanine shown as a future fit-out, decide whether it is in the application or not, because a mezzanine on the drawings is a mezzanine in the calculation.

Voids over double-height spaces are excluded at the upper level but the floor below them counts in full. The error people make is the reverse, excluding the lower floor as well on the basis that the space is double height.

Plant rooms, substations within the building, bin stores inside the envelope and cycle stores inside the envelope all count. Moving a bin store or a cycle store outside the building line changes the figure, and on a scheme near a charging threshold that is a design decision with a price attached.

Existing floorspace and the in-use test

The deduction for existing floorspace is normally the largest single number in the calculation, and it has a test attached that has nothing to do with measurement.

Floorspace in a building on the relevant land can be deducted where that building 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 the chargeable development. Two words carry the weight. Lawful, so a use in breach of condition or without permission fails. And continuous, so six separate months across three years is not six continuous months.

The authority may treat a building as not in use where the evidence is insufficient, and the practical burden sits with the applicant. Business rates records, council tax records, utility bills showing consumption, tenancy agreements with corroborating payments. Collect them before the building comes down, because they are much harder to obtain afterwards.

The building also has to be on the relevant land. A structure outside the red line on the application drawings is not deductible however close it stands, which makes the red line a levy decision as well as a planning one.

On a conversion of an older building the deduction often decides whether the scheme works at all, which is why it features so heavily in viability on a listed building conversion.

Demolition and the deduction

Timing, and only timing.

A building demolished before planning permission is granted is no longer on the land and no longer in use, so its floorspace cannot be deducted. A building standing at the date permission first permits development, which satisfied the in-use test in the preceding three years, can be deducted even though it is demolished afterwards.

Clearing a site early to make it marketable can therefore cost more in levy than it gains in land value. Where demolition is needed for site investigation or security, record the floorspace and the use evidence first, with a measured survey and dated photographs, and take advice on the sequencing before anything comes down.

Partial demolition needs the retained and demolished areas measured separately and shown on a plan, because the calculation treats them differently and an aggregate figure invites a challenge.

Presenting the calculation on the form

The additional information form submitted with the planning application asks for existing floorspace, floorspace to be retained, floorspace to be demolished and proposed floorspace, by intended use.

Submit a measured schedule alongside it rather than bare totals. What a collecting authority can process without querying:

1. A floor by floor table, existing and proposed, with areas in square metres to one decimal place.
2. A statement of the convention used and the plan references the areas were measured from.
3. Separate columns for retained, demolished and new floorspace.
4. A note of any area excluded, with the reason, including low headroom areas and open balconies.
5. The in-use evidence for each existing building, listed by building and by document.
6. Use class or intended use for each area, since charging schedules set different rates by use.

Then make sure the same figures appear in the appraisal. A levy line in a viability appraisal that does not reconcile with the form is the fastest way to lose a reviewer's confidence, as discussed in viability appraisal outputs, and the same applies to the unit areas underlying space standards and accessible housing as appraisal inputs.

Where authorities challenge a figure

The challenges are predictable, which makes them avoidable.

Measuring to the external face rather than the internal face, which inflates nothing and understates nothing but produces a figure the authority recalculates anyway. Omitting internal circulation, stairwells and lift wells. Omitting plant rooms and risers. Excluding internal car parking. Claiming a deduction for a building outside the red line. Claiming a deduction for a building demolished before permission. Inconsistency between the architect's area schedule, the form and the drawings. And a single aggregate figure with no floor by floor breakdown, which an authority will simply remeasure from the drawings.

If a liability notice arrives with a figure you dispute, the route is to request a review by the collecting authority within the short period the regulations allow, and then to appeal the chargeable amount to the appointed valuer. Both windows run from the date of the notice, so a notice that sits unopened for a fortnight has already cost you options. A Viability Assessment prepared with the measured schedule attached gives you the evidence for either step without further work.

Practices preparing these schedules regularly can draft the supporting documents on the Professional tier and keep the area schedule and the narrative in one place. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval.

This is general guidance, not legal advice; check the current text of the Community Infrastructure Levy Regulations 2010 as amended and confirm the position with your collecting authority or a qualified professional before relying on it.