How is the building area calculated?

Floor areas for energy assessments are based upon Gross Internal Area (GIA) as defined in the current RICS Code of Measuring Practice.  Further information can be obtained here.

In order to complete a Display Energy Certificate, the GIA of the building must be accurately determined.  The energy assessor is responsible for verifying any value provided and for ensuring that it is accurate.  The default fallback position is that the energy assessor will be required to measure the building to determine the GIA.

However, it is recognised that the buildings requiring DECs are often large and complex.  As such, it is likely that other professionals are involved in building management and a GIA for the building may be available from other means.  This can avoid the lengthy and costly process of the energy assessor measuring the entire building e.g. a large college or acute hospital.  Therefore, DEC assessors are authorised to accept the following sources of information providing they take appropriate sample site measurements to verify the figures used are reasonable.  The acceptable secondary sources are:

  1. Measurements from scaled drawings: – This method can be used where professionally produced scaled plans are available for the building.  It is most useful when measurement layers have been turned on.  Particular care must be taken to ensure that measurements shown are Gross and not Net of internal walls etc.
  2. Areas from a formal Asset Register or similar: – This is often the simplest method where a suitable document exists but the energy assessor must be able to demonstrate that this is a formal document that has been professionally produced.  Documents in electronic format are acceptable.  Particular care must be taken to ensure the figures used are clearly identifiable, accurate, current and Gross not Net.  It is not unusual for these registers not to be updated to reflect building changes or for inaccurate or Net figures to be input in error.
  3. Information from an Architect or Surveyor who has reason to know: – Where an Architect or Surveyor who is registered with an appropriate professional body has undertaken work relating to the building they may have established the GIA.  This value can be used with appropriate supporting evidence.  This is also the justification used to accept a GIA printed on professionally produced scaled drawings of a building.

Uniquely for DEC’s, it is sometimes acceptable to use alternative measures of floor areas for some specific types of building.  However, these exceptions are very limited and the software used to produce the DEC converts these values to GIA before calculating the building rating.  The conversion is very conservative and so the rating will nearly always be considerably worse if these values are used that if the GIA is used directly.  Additionally, the GIA is normally available when a suitable source of these values is provided and so using them is not advisable.

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