What are SAP calculations?
SAP stands for Standard Assessment Procedure. A SAP calculation models the energy performance of a home using its drawings, construction details and building services. It helps show whether a proposed new home meets the energy requirements of Building Regulations and provides the assessment behind its Energy Performance Certificate (EPC).
Your assessor builds a model of the dwelling in approved software. Rather than looking at insulation or heating on its own, the calculation brings the whole design together: the size and shape of the home, its windows, construction, junctions, heating, hot water and ventilation.
The result is more than a single score. For a new home in England, the assessment checks emissions, primary energy and fabric energy efficiency against the relevant targets. A good EPC rating alone does not tell you that all those checks pass.
Does your project need SAP calculations?
The assessment you need depends on whether you are creating a new home, converting an existing building or extending one. These projects do not all need the same calculations.
New builds
A new dwelling needs an energy assessment before construction and an updated assessment at completion. Starting while you can still change the specification makes it easier to resolve problems. Our new build SAP service covers the Design and As Built stages.
Conversions
Turning a barn or commercial building into a home, or dividing a house into flats, is different from adding a room to an existing home. These projects can involve a material change of use, and their energy requirements differ from those for a newly constructed house. See our conversion assessment service for the work and documents involved.
A loft or garage conversion that remains part of your existing home does not normally need a full SAP calculation. Insulation, windows and building services still need to meet the applicable requirements.
Extensions
Not every extension needs SAP. A design may meet the standard fabric and opening requirements without a whole house assessment. Where there is additional glazing, an Area Weighted U-value calculation may provide the flexibility needed. A whole house SAP comparison considers improvements to the existing home as well as the extension.
Our extension calculations page explains the three methods so you can identify the right service before booking.
The detailed examples in this guide relate to England. Requirements differ across the UK, so tell your assessor where the project is and which standards apply.
Design SAP and As Built SAP: what is the difference?
The Design assessment checks what you intend to build. The As Built assessment checks the finished home using its final specification and supporting evidence.
Design SAP: before construction
The Design assessment uses the proposed construction and specification. It shows whether the new home can meet the applicable energy targets and allows your assessor to test changes before you order materials or equipment.
For a new build in England, the documents can include the Design SAP calculations, a Building Regulations England Part L (BREL) report and a Predicted Energy Assessment (PEA). The PEA indicates the expected energy rating; it is not the completed home's final EPC.
As Built SAP: at completion
The completed assessment uses what was actually built. Your assessor checks the final specification and supporting evidence, updates the calculation and prepares the completion documents. For a new build, this includes the As Built BREL report and the final EPC.
Tell your assessor about changes during the build. A different window, insulation product, heat pump or ventilation system can change the result. It is better to check a substitution before installation than discover its effect at completion.
Our Design SAP and As Built SAP comparison explains the stages in more detail. The documents for conversions and extension comparisons are different, so use the relevant service page to check what is included.
What affects a SAP calculation?
Two homes with the same floor area can have very different results. Their shape, exposed surfaces, orientation and specification all affect the model. The main inputs include:
- Walls, roof and floor: their areas, construction and calculated U-values.
- Windows and doors: sizes, positions, whole product U-values and the solar energy passing through the glass.
- Thermal bridging: additional heat loss at junctions such as window surrounds and wall to floor connections.
- Air tightness: the proposed target at design stage and measured result at completion.
- Building services: heating, hot water, controls and ventilation, including the performance of the specified equipment.
- Renewables: systems such as solar PV and their contribution to the assessment.
Specific product information matters. A named heat pump or ventilation unit may perform differently from the generic assumption used before the equipment was selected. The same applies to insulation conductivity and glazing specifications.
SAP uses standard assumptions about how a home is occupied. It is a consistent assessment of the building, not a forecast of the particular occupier's energy bills.
What happens if your SAP calculation fails?
A first assessment that falls short is a reason to investigate the design, not immediately order thicker insulation. First establish which target is failing and check that the model reflects the correct drawings, construction details and equipment.
Then compare realistic changes. Better junction details, a revised glazing specification or more accurate equipment data may help. Other projects need changes to insulation, heating or renewable generation. The useful question is which improvement gets the required result for the least disruption and cost.
Different measures affect different targets. Solar PV can help some energy and emissions results, but it does not fix a fabric energy efficiency shortfall. Your assessor should explain what is causing the problem before recommending a solution.
In one project, reviewing an excessive insulation specification helped our client achieve all three SAP targets with much less insulation. Read how we estimated an £11,792 material saving.
Advice and reasonable revisions are included in our assessments. Our practical SAP checklist covers things to keep track of as the project develops.
What information does your SAP assessor need?
Start with your latest drawings and the specification you have. The main information is:
- Dimensioned floor plans, elevations and sections.
- A site plan showing the dwelling's orientation.
- Wall, roof and floor construction details, including insulation products and thicknesses.
- Window and door sizes, U-values and glazing specifications.
- Heating, hot water and ventilation details, including product models where chosen.
- Thermal bridging details and any renewable system proposals.
You do not need every product chosen before you contact an assessor. Explain what is still undecided so the options can be assessed rather than treated as final selections.
At completion, the assessor needs evidence of the final construction and services. For new builds this includes relevant photographs, air test results and commissioning records. Ask for the completion checklist early so your builder can collect evidence before work is covered up.
For multiple plots, identify the house types, plot references and differences between dwellings. Shared drawings are useful, but orientation, exposed surfaces and specification can still differ.
How much do SAP calculations cost, and how long do they take?
The fee depends on the assessment, dwelling size and number of plots. A repeated house type on a development is a different amount of work from an individual home with a bespoke specification.
When comparing quotations, check which stages are covered, whether the final EPC is included and what happens if the specification changes. A Design SAP price and a package covering both stages are not the same purchase.
See our current SAP calculation prices for the available packages, or request a development or bespoke project quote.
Our standard new-build assessment turnaround is 4 working days once we have the drawings and specification needed to start. Completion timing also depends on receiving the final evidence. If you have a deadline, tell us before booking so we can confirm availability.
SAP calculation questions
SAP, EPC and SBEM describe different assessments or documents. These answers explain the distinctions and who needs to be involved.
Is a SAP calculation the same as an EPC?
No. SAP is the detailed energy assessment. The EPC is a certificate produced from the completed assessment. An existing home being sold or rented is usually assessed using RdSAP, which uses a different data collection approach.
Can I do my own SAP calculation?
You can explore your design using energy tools, but the compliance assessment and final EPC need an appropriately qualified and accredited assessor using approved software.
Does each dwelling need its own assessment?
Yes. Similar plots can share drawings and specification information, but each dwelling needs to be represented correctly. Its orientation, exposed walls, roof or floor can affect the result.
Do I need a site visit?
SAP assessments are normally completed from drawings, specifications and supporting evidence. Site testing, such as the air test, is separate from the desk-based SAP calculation.
Is SAP the same as SBEM?
No. SAP assesses dwellings. SBEM is used for many non-domestic buildings. A mixed-use project may need different assessments for its residential and commercial parts.
Where to find the official guidance
The government's Standard Assessment Procedure guidance explains the methodology. For the energy requirements in England, see Approved Document L.
The relevant standards depend on the project and its application dates. Your assessor should confirm the applicable requirements before starting.