Reduced Data SAP (RDSAP) is the Government-approved survey system used to generate Energy Performance Certificates for existing homes. RDSAP evolved from the SAP methodology for assessing new build properties.

Because a full SAP requires many data items that cannot be seen in a survey of an existing house, several energy rating companies each created different systems to assume the missing data. RDSAP was created by reviewing these existing proprietary systems - and developing a new system combining the best aspects of each of them.

RDSAP requires assessors to collect data about the most important factors that would affect the SAP rating. RDSAP software then calculates a full SAP rating using derives (rather than inputted) values for the missing data. This calculation gets to within 5% of a full SAP assessment, but in a far shorter time.

RDSAP produces an energy performance certificate and advice report that places the SAP rating on an A-G scale similar to the labels on white goods. It also provides an Environmental Rating of the property in terms of CO2 emissions, in a similar format. The report also provides an indication of costs for space and water heating and lighting, (based on the SAP) and improvement recommendations.

You can see an sample Energy Performance Certificate based on the RDSAP methodology here.

Work to develop the RDSAP methodology started in 2003 and was split into various phases: