What is the Difference Between SAP and RdSAP?

If you are looking to obtain an Energy Performance Certificate (EPC) for your dwelling, you may have come across a number of acronyms that make understanding energy assessments just short of a riddle. Both SAP and RdSAP calculations serve the same purpose, in this blog, we explain these two methods of obtaining an EPC, and explore the differences between them.

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What are SAP Calculations?

Since 1995, SAP calculations are necessary for all new residential dwellings under Part L of the Building Regulations. SAP stands for Standard Assessment Procedure, the official method approved by the government for calculating the energy performance and assessing the energy rating for new dwellings. 

The main purpose of SAP calculations is to ensure it meets current Part L Building Regulations and to produce an Energy Performance Certificate (or EPC) for the dwelling. This is done by measuring the Dwelling Emission Rate (DER) and Dwelling Fabric Energy Efficiency (DFEE) against the predetermined Target Emission Rate (TER) and Target Fabric Energy Efficiency (TFEE). 

The rating produced by a SAP calculation is presented as a figure between 1 and 100+. The higher the energy rating of the dwelling, the lower the energy costs and emissions associated with it are, with a rating of 100 representing zero energy costs. 

You can learn more about what  a good SAP rating is in our recent blog

What are RdSAP Calculations?

RdSAP stands for Reduced Data Standard Assessment Procedure, and it is the official method for producing Energy Performance Certificates for existing residential dwellings. This procedure was developed in 2005 using the framework of SAP, and it is essentially a simplified version of the latter.   

The main purpose of RdSAP calculations is to enable surveyors to gauge the energy efficiency of a dwelling when SAP calculations are not available, or when the existing EPC has expired. SAP calculations should be available for any new dwellings built after the relevant date in your local area, respectively  6 April 2008 in England and Wales, 30 September 2008 in North Ireland, or 1 May 2007 in Scotland.

What is the Difference Between SAP and RdSAP Calculations?

As we have seen above, SAP and RdSAP calculations serve the same purpose: to produce EPC ratings for residential dwellings. RdSAP is also derived from the same framework as SAP calculations, albeit simplified. The main difference between the two is that SAP calculations are used to ensure compliance with Part L of Building Regulations, and to determine EPC for new dwellings, extensions, and changes of use, whilst RdSAp calculations are used to determine EPC ratings for existing dwellings. 

In what follows, we’ll take a more in-depth look at the differences between SAP and RdSAP calculations.

SAP vs RdSAP: Why Calculations are Required

SAP calculations are required for all new dwellings by Part L (England & Wales), Section 6 (Scotland), Part F1 (Northern Ireland) of Building Regulation standards. SAP calculations are used to measure Carbon Emissions and Energy Efficiency in order to prove that new builds comply with Building Regulations and can be signed off as marketable dwellings. 

RdSAP calculations, on the other hand, are required as part of The Energy Performance of Buildings Regulations. Their main purpose is to identify possible energy improvements for existing dwellings. They are also required in order to sell or rent dwellings where an SAP calculation is not available, and to update an EPC where more than 10 years have passed since it was last calculated.

SAP vs RdSAP: When are Calculations Undertaken?

SAP calculations for new buildings, and changes of use are undertaken twice: first at the design stage and a second time once the build is completed. SAP calculations for extensions are typically only undertaken once.

At the design stage, assessors will produce calculations to assess whether the building is DER and DFEE compliant, create a predicted energy assessment, and approve or recommend modifying the design on the basis of these calculations.

Once the building is complete, SAP calculations are used to reassess the construction details and produce an up-to-date EPC rating for the dwelling. 

RdSAP calculations are used to create EPC ratings for existing dwellings that have not been built under new building regulations. Dwellings built after 6 April 2008 in England and Wales, 30 September 2008 in North Ireland, or 1 May 2007 in Scotland should have an EPC certificate produced using full SAP methodology. A RdSAP calculation may also be used to produce an updated EPC if the property is being sold or rented and the original EPC has passed its expiration date of 10 years.  

SAP vs RdSAP: How are Calculations Undertaken?

SAP calculations are desk-based exercises carried out by qualified On Construction Domestic Energy Assessors (OCDEAs). In order to produce an accurate EPC rating, as much information as possible should be supplied at the design and build stages. This includes site drawings, construction plans and methods, full HVAC (heating, ventilation and air conditioning) specifications, and any renewable technologies and cooling systems used. 

RdSAP calculations, on the other hand, are carried out by a Domestic Energy Assessor (DEA), who will visit the property in order to obtain information about features such as the type of dwelling, its age, measurements, construction method, method of insulation, the type of water heating systems used, types of windows, and any renewable or energy-saving technologies used. The surveyor uses this data, accompanied by pictures and documents where relevant, to lodge an EPC request on behalf of the client. 


While both SAP and RdSAP calculations are used to produce an EPC certificate for a dwelling, differences arise on how, when, and why they are produced. 

SAP calculations are used to produce EPC ratings for new dwellings, certain extensions and changes of use, in order to comply with Building Regulations. They are carried out based on technical data by an On Construction Domestic Energy Assessor at both the design and build phase of the project.

RdSAP calculations are used to produce EPCs for existing dwellings and to produce new EPC ratings every 10 years if the property is being sold or rented. These calculations are based on a property survey carried out by a Domestic Energy Assessor.

SAP Calculations With RS Energy

We know that navigating energy calculations can be stressful and confusing. At RS Energy, we provide SAP calculations and tailored consulting services that help you achieve compliance efficiently and cost-effectively. Our experienced and accredited SAP assessors work with a variety of properties and a wide range of clients across England and the UK. 

Get in touch today to find out more about how RS Energy can help ensure that your development meets Building Compliance requirements.