How CIL is calculated

CIL is charged against the net additional floorspace. Once the total chargeable floorspace is determined, the liability is then calculated in accordance with the CIL Regulations (Regulation 40 and Schedule 1). It is only at this point that demolished / converted floorspace is deducted from the CIL ‘liable’ floorspace leaving the CIL ‘chargeable’ floorspace.

Chargeable floorspace

CIL is calculated using Gross Internal Area (GIA). The RICS Code of Measuring Practice definition of Gross Internal Area is used in all instances. The following is a technical summary of what is included and excluded when calculating the GIA.

Chargeable floor space includes:

• areas occupied by internal walls and partitions
• areas with a headroom of less than 1.5m
• columns, piers, chimney breasts, stairwells, lift-wells, other internal projections, vertical ducts, and the like
• atria and entrance halls with clear height above, measured at base level only
• pavement vaults
• garages
• conservatories  
• internal open sided balconies, walkways, and the like
• structural, raked or stepped floors are treated as a level floor measured horizontally
• horizontal floors with permanent access below structural, raked or stepped floors
• corridors of a permanent essential nature (e.g. fire corridors, smoke lobbies, etc)
• areas in the roof space intended for use with permanent access (BCIS)
• mezzanine areas intended for use with permanent access
• lift rooms, plant rooms, fuel stores, tank rooms which are housed in a covered structure of a permanent nature, whether or not above main roof level
• service accommodation such as toilets, toilet lobbies, bathrooms, showers, changing rooms, cleaners’ rooms and the like
• projection rooms
• voids over stairwells and lift shafts on upper floors
• loading bays.

Chargeable floor space excludes:

• perimeter wall thickness and external projections
• external open-sided balconies, covered ways and fire escapes
• canopies
• voids over or under structural, raked or stepped floors
• greenhouses, garden stores, fuel stores and the like in residential property

Separate buildings – GIA excludes the thickness of external walls but includes the thickness of all internal walls.  A residential garage attached to a house, regardless of access point, is NOT a separate building.  Similarly, semi-detached or terraced dwellings are measured as one building.

Internal face – means the finished surface provided by the initial builder, e.g. brick / block work or plaster coat applied to the external walls, not the surface of internal linings or skirtings installed by occupier.

Existing buildings to be demolished or converted - The GIA of existing buildings within the chargeable development that are going to be either converted or demolished may be deducted from the total CIL liable GIA if:

• it is not greenhouses, garden stores, fuel stores and the like in residential property; and
• the building is permanent and substantial; and
• the building going to be demolished has been in continuous lawful use (meaning its lawful purpose) for 6 months or more in the last 3 years on the date of approval of the permission; or
• the existing building will continue to be used for its lawful purpose without a change of use.

Please note: Floorspace due to be demolished / converted must be standing, and contained within the red line of the development, on the day that planning permission is approved.

Proposed converted / demolished floorspace will not be considered as eligible for deduction unless evidence is submitted confirming its in-use status.

A building must be in-use for its lawful purpose - it is not sufficient that the building has a lawful use to which it could be put.

As an example, stables must be in-use as a building in which livestock (especially horses) are kept, not used as general storage.

If the site has a mixture of use classes contained within its proposed development, any eligible demolished floorspace will be deducted from any new floorspace via apportionment.

Retrospective applications

Where a development has been undertaken without permission and retrospective planning permission is sought to regularise the development, any CIL payment will be due immediately upon the grant of planning permission and will incur late payment interest from the day after the planning decision is issued to when the CIL is paid in full. 

Retrospective development cannot be eligible for any exemption or relief from CIL.