The Reduced Data SAP (RdSAP) is the government approved methodology for assessing the energy performance of existing dwellings. The SAP assessment methodology has applied in the energy rating calculations of millions of homes across the UK since 2007. RdSAP is the calculation tool behind the Energy Performance Certificates (EPC). EPCs are required by law if you market a property for sale or to let. Currently, an EPC costs around £100. However, this is soon to change.

Later in 2024 the Department for Energy Security & Net Zero will be introducing a new assessment methodology for the energy rating of existing dwellings called RdSAP10. This is the first major update of the current RdSAP method since 2007. The purpose of the change is to address issues related to inaccuracies of the EPC ratings. The new assessment will mean a far more detailed survey on site and more data input in the calculation software.

Here are some of the changes homeowners, landlords and estate agents should expect to see once the new energy rating method is implemented:

1. Old houses will rate lower

When assessing a home the age band helps assign U-values to the walls, floors and roofs. In the current SAP assessment old houses tend to rate low, whereas new homes tend to rate high. It is expected that in the new SAP, old houses will rate even lower and new houses will rate even higher.

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2. Assessors will need to measure all windows

In the current methodology, assessors do not measure windows individually as RdSAP assumes size of openings, such as doors and windows, based on age of build. In the new methodology, the assessors will look at each window and door individually. All external doors and windows will be measured and assessed for glazing thickness and orientation.

3. More work in the attic

RdSAP10 will mean that assessors will have to measure gable walls and party walls in roof conversions or rooms in roofs. This will assign default insulation values in roof rooms based on the age band of the building. Inevitably, the process will involve a lot more data collection and calculations.

4. U values will be the key metrics

U values indicate the thermal resistance of a wall, floor or ceiling. U-values of construction elements are determined within the software from the type of structure, date of construction and thickness of insulation.

5. Mechanical ventilation will be considered

In the last few years building regulations gave emphasis on heat loss preservation and effective insulation. This approach, especially in new housing, created a lot of overheating issues. Mechanical ventilation will now be considered for recirculating the heat, as well as allowing homes to "breathe".

6. Heat pumps may now be recommended as a main heating system

The current SAP calculations rate fuels like oil and mains gas higher than electricity. For this reason heat pumps were never recommended when a dwelling was suitable for oil or mains gas boilers. This will now be different as the new SAP methodology will assess the dwelling’s suitability for air source heat pumps, especially if these can feed from electricity produced by photovoltaic panels.

7. Expect to pay more

The new EPC assessment methodology is more detailed than the current assessment as it tries to provide EPC assessors with the correct assessment values while ensuring homeowners have the right recommendations to improve their energy rating. Therefore, homeowners should expect to pay more for their EPC when the new methodology comes into effect later in 2024.

The government have suggested that the likely release of the new energy rating process will be mid-summer 2024.