Calculating the CIL charge

Floorspace is measured according to the Royal Institute of Chartered Surveyors (RICS) Code of Measuring Practice for Gross Internal Area (GIA). Residential floorspace includes new dwellings, extensions, conversions, garages or any other buildings ancillary to residential use. 

Gross Internal Area (GIA)

GIA is the internal area of the building, and should include: 

  • rooms 
  • circulation and service space such as lifts and floorspace devoted to corridors and communal entrances 
  • toilets 
  • storage 
  • basements 
  • ancillary floorspace (eg underground parking).

The following will be excluded from CIL liable floorspace: 

  • buildings "into which people do not normally go" (including buildings into which people go intermittently for the purpose of inspecting or maintaining fixed plant machinery) 
  • conversion of one dwelling into two or more dwellings (including conversion from a single house into flats).  This only applies when the development is a conversion only and no extensions are planned i.e no increase to the GIA 
  • 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.

Floorspace resulting from demolition or change of use can be deducted from the CIL charge if: 

  • it has been in continuous lawful use for at least six months in the three years prior to a development being permitted
  • on the day planning permission is granted the floor space of the existing building exists
  • the existing building is demolished before completion.

Please note: The CIL charge per sqm applies to the entire GIA for new dwellings and extensions exceeding 100 sqm, meaning that the whole floorspace is charged and not just the amount over 100 sqm. 



An attic will be included in GIA if there is access to it via a fixed, permanent stairway or ladder, but not if by a pull-down ladder. 

Car Ports 

A car port will be defined as a garage and included in GIA if it is walled on one or more sides and is covered by a roof. The walls do not have to go to roof height, as they can support the roof by piers. If the car port does not have at least one wall it will be classed as a canopy and will not be included in GIA. 


porches on dwelling houses even though the front or side may be open will be included in the GIA. 

Flats to single dwelling 

The conversion of flats to a single dwelling will be CIL liable. This may not result in an increase in the internal space due to deductions from the retained buildings, but the calculations need to be done. 

Greenhouses, garden stores, fuel stores, and the like within the curtilage of residential properties 

These will be excluded from the GIA calculation. 

Ancillary buildings

Buildings which are large enough for a person to enter including garages, conservatories games rooms, offices and gyms will all be included in the GIA calculation.


Ancillary buildings which are self-contained units i.e. either a studio or a 1bed unit are considered Annexes for CIL purposes and are CIL liable even if the floor space is under 100 m2.

Low Headroom 

Areas with headroom of less than 1.5m are not chargeable under CIL except if this area is located under the stairs. 

Swimming Pools 

External swimming pools under cover will be classed as other buildings ancillary to residential use, are chargeable and would be included in GIA. External swimming pools with no structure over are outside the scope of CIL and not chargeable. 

Share this page

Share on Facebook Share on Twitter Share by email


Print this page