SAP Calculations for New Dwellings

What This Requirement Covers

The Standard Assessment Procedure (SAP) is the Government's methodology for calculating the energy performance of dwellings. SAP is used to demonstrate compliance with Part L of the Building Regulations for new dwellings and to generate the Energy Performance Certificate (EPC). The current version is SAP 10.2 (used with the 2021 Part L).

Key Requirements

What SAP Calculates

SAP calculates

1. Dwelling Emission Rate (DER): The estimated annual CO2 emissions in kg/m²/year
2. Target Emission Rate (TER): The maximum permitted CO2 emissions (based on a notional dwelling of the same size and shape)
3. Dwelling Fabric Energy Efficiency (DFEE): The energy demand from the building fabric in kWh/m²/year
4. Target Fabric Energy Efficiency (TFEE): The maximum permitted fabric energy demand
5. Dwelling Primary Energy Rate (DPER): The primary energy consumption in kWh/m²/year
6. Target Primary Energy Rate (TPER): The maximum permitted primary energy consumption
7. EPC rating: The overall energy efficiency rating on a scale of A to G

To comply with Part L, the DER must not exceed the TER, the DFEE must not exceed the TFEE, and the DPER must not exceed the TPER.

Key Inputs

1. Building geometry: Floor area, volume, number of storeys, orientation
2. Fabric performance: U-values for all external elements (walls, roof, floor, windows, doors)
3. Thermal bridging: Psi-values at all junctions (from ACDs or individual calculations)
4. Air permeability: Design value (and subsequently the tested value)
5. Heating system: Boiler efficiency, heat pump SCOP, or other heating system performance
6. Hot water system: Cylinder size, insulation, and heat source
7. Ventilation system: Type (natural, mechanical extract, MVHR) and performance
8. Renewable energy: Solar PV, solar thermal, or other on-site generation
9. Lighting: Proportion of low-energy (LED) lighting

The SAP Assessor

1. SAP calculations must be carried out by an accredited SAP assessor
2. Accreditation is provided through approved schemes (e.g., Elmhurst, Stroma, ECMK, Quidos)
3. The assessor produces both the Design Stage SAP (used during the Building Control application) and the As-Built SAP (produced after construction, using the actual air test result and any changes from the design)
4. The assessor also produces and lodges the EPC on the national register

SAP vs RdSAP

1. SAP (full SAP): Used for new dwellings and major renovations; requires detailed input from drawings and specifications
2. RdSAP (Reduced Data SAP): Used for existing dwellings being sold or let; based on a site survey with default assumptions for unknown elements

Practical Compliance Tips

1. Engage the SAP assessor at the earliest design stage; they can advise on the most cost-effective compliance route
2. Run the SAP calculation iteratively as the design develops; small changes in fabric specification or heating system can significantly affect the result
3. Provide the SAP assessor with accurate U-value calculations, thermal bridging details, and heating system specifications
4. The air permeability test result is entered into the as-built SAP; a better-than-expected result improves the DER, while a worse result may cause non-compliance
5. Solar PV is often the most cost-effective way to close a compliance gap in the SAP calculation
6. Keep the design stage and as-built SAP worksheets for the building file; Building Control requires both
7. Plan for the SAP assessment timeline in the construction programme; the as-built SAP and EPC must be produced before the completion certificate is issued