The Standard Assessment Procedure, commonly abbreviated to SAP, is used to assess the energy performance of new dwellings in the United Kingdom.
For existing dwellings the Reduced Data Standard Assessment Procedure is used (RdSAP). This is based around essentially the same model but requires less data about the building to be available. The assessment is therefore based on the data obtained by an accredited assessor during a visual survey conducted during a site visit.
Both methodologies were originally based on research undertaken by the Building Research Establishment (BRE). In particular, they developed a Domestic Energy Model known as BREDEM which was used to develop the government approved methodologies.
Further information can be found on the Gov.UK website.