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The Energy Performance Certificate is measured using a system called the Reduced Data Standard Assessment Procedure (RDSAP). This system was developed from the Standard Assessment Procedure (SAP). In order to provide a like for like comparison between different properties regardless of its inhabitants, both the RDSAP and the SAP make certain assumptions about the way the occupants use the property. This is called ‘standard occupancy’ and it ensures that the properties rating is not biased by the occupiers demand for energy. ‘Standard occupancy’ assumes that the house will be heated for 9 hours a day in the week and 16 hours a day at weekends, with the living area heated to 21°C and the rest of the house to 18°C.

The difference between RDSAP and SAP is that the former requires much less data input than the latter and can therefore be done relatively quickly and cost effectively. The full SAP is often calculated before a building is constructed and ensures compliance with current building regulations relating to energy efficiency. It requires knowledge of building materials that may not be able to be seen after the house has been constructed. RDSAP was developed to enable surveyors to gauge the energy efficiency of existing properties. Like a valuation or even a structural survey, the RDSAP survey is a non-intrusive survey. This means that the surveyor is not allowed to disturb the fabric of the building and must not cause any damage.

Any furniture and belongings should not be moved without permission. This clearly limits the information that is obtainable, however recent surveys of houses that have had both the SAP and the RDSAP calculations seem to suggest that the RDSAP system produces very similar results to the full SAP system. If the inspection of certain areas of the building is restricted, the DEA is required to note them on their site notes and input the restrictions into their RDSAP software as ‘no access’. The software will then assume that the property meets the minimum building regulation standards of the time of construction and generate an EPC accordingly.

It is always wise to allow the DEA as much access as possible as the software will default to the lowest levels of efficiency possible within building regulations for the date of construction of the property if an area cannot be inspected. For example if the roof insulation cannot be inspected the system will often assume (depending on the houses age) that there is none present. If the roof is insulated but the DEA could not verify this it will be recorded as ‘no access’ and your EPC will be significantly affected.