Our assessors use approved SAP modelling software to compare the performance of a proposed dwelling design against a series of targets that are based on the building regulations in force at the time. Such targets include the Fabric Energy Efficiency Standard, maximum permitted U-values and summer overheating requirements, but the best known is the carbon emissions target whereby the DER (Dwelling CO2 Emission Rate) is compared with the TER (Target CO2 Emission Rate) – DER must be less than TER for compliance.