CIL liability is based on the “chargeable area” which is calculated using the formulas set out in Regulation 40 of the CIL Regulations 2010 (as amended). These require the measurement of the Gross Internal Area (GIA) floorspace of the proposed CIL liable development and the GIA floorspace of any existing ‘in-use’ building which is to be demolished.

An 'in-use' building is 'A building that has been in ‘lawful use’ for a continuous period of at least 6 months within the period of 3 years, ending on the day planning permission first permits the charge for development'.

GIA is the area of a building measured to the internal face of the perimeter walls at each floor level and includes*:

• Areas occupied by internal walls and partitions;
• Columns, piers, chimney breasts, stairwells, lift-wells, other internal projections, vertical ducts;
• Atria and entrance halls, with clear height above, measured at base level only;
• Internal open-sided balconies, walkways;
• Structural, raked or stepped floors are to be 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);
• Mezzanine floor areas with permanent access;
• Lift rooms, plant rooms, fuel stores, tank rooms, which are housed in a covered structure or a permanent nature, whether or not above the main roof level;
• Service accommodation such as toilets, toilet lobbies, bathrooms, showers, changing rooms, cleaner’s rooms;
• Projection rooms;
• Voids over stairwell and lift shafts on upper floors;
• Loading bays;
• Areas with a headroom of less than 1.5m;
• Pavement vaults;
• Garages;
• Structures, such as car ports with a solid roof even though one or more sides is open;
• Porches on dwelling houses even though the front or side may be open;
• Conservatories;
• Basements, underground and covered car parking areas.

The following are excluded from CIL liable floorspace*:

• Perimeter wall thicknesses 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 (sheds), fuel stores and the like, in residential properties;
• A pergola with no roofing material;
• Free-standing solar panels.

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*Note that each Council may differ in what is included and excluded in CIL liable floorspace and therefore it is always advisable to check with them prior to planning any works.