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Calculating the Community Infrastructure Levy 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)

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

  • rooms 
  • garages
  • circulation and service space such as lifts and floorspace devoted to corridors and communal entrances 
  • toilets 
  • storage 
  • basements 
  • ancillary floorspace, such as underground parking

The following will be excluded from Community Infrastructure Levy (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, for example no increase to the gross internal area
  • 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 Community Infrastructure Levy 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 Community Infrastructure Levy charge per sqm applies to the entire gross internal area 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 the gross internal area 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 the gross internal area 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 the gross internal area. 


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

Flats to single dwelling 

The conversion of flats to a single dwelling will be Community Infrastructure Levy 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 gross internal area 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 gross internal area calculation.


Ancillary buildings which are self-contained units, such as either a studio or a one bedroom unit are considered Annexes for Community Levy Infrastructure purposes and are Community Infrastructure Levy liable even if the floor space is under 100 m2.

Low Headroom 

Areas with headroom less than 1.5m are chargeable under Community Infrastructure Levy.

Swimming pools

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

Strategic sites

When measuring large developments, party walls between terrace houses and flats are included in the calculation of the gross internal area, as are the party walls between houses and attached garages.

Communal areas including floorspace devoted to corridors and communal entrances are also included in gross internal area. 

Where bin and cycle stores that are permanent structures are included, their floor space will also be included in the gross internal area. 

If you have any questions regarding measuring the gross internal area for strategic sites, please contact us to discuss.

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