Calculating the Gross Internal Area

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. Generally, any structure with 3 walls and a roof can be a building with internal floorspace and is therefore chargeable under CIL.

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

  • area occupied by internal walls and partitions, chimney breasts, stairwells, lift wells, vertical ducting and the like
  • all rooms including conservatories
  • atria, entrance halls, circulation space, corridors and communal entrances
  • toilets, toilet lobbies, bathrooms, showers, changing rooms, cleaners’ rooms and the like
  • garages and loading bays
  • storage
  • plant rooms, fuel storage, tank rooms and the like, including those on the roof
  • basements
  • internal open sided balconies, walkways and the like
  • 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)
  • 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; and
    • 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.


Attics – 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.

Caravans – A caravan is a temporary structure which is sited on land and does not constitute development in its own right. There is therefore no floorspace associated with the siting of a caravan.

Car Ports – A car port will be defined as a garage and included in GIA if it is walled on three 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 there are two walls or less, it will be classed as a canopy and will not be included in 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 – As a general rule, ancillary buildings which are large enough for a person to enter will be included in the GIA calculation. A building/structure into which people do not normally go/cannot get into, will be excluded from the GIA calculation.

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

Mezzanines – The insertion of a mezzanine floor in an existing building (eg a barn conversion) is outside the scope of CIL and is not chargeable, unless the insertion of the mezzanine is part of other proposed operational development (eg the insertion of windows/doors, re-roofing). The insertion of a mezzanine floor in a new building is chargeable and should be included.

Pole Barn – A pole barn without three or more walls (can be partial walls in height) will not count as a building and cannot be deducted from gross new floorspace if it is to be demolished or retained as part of a new development.

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.