Help & Support
SAP Calculation FAQs
Clear answers to the most common questions about SAP Calculations, extensions, conversions, overheating (Part O) and thermal bridging. If you do not see your question here, just get in touch.
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General SAP Questions
These questions cover the basics of SAP Calculations, who needs them, and how the process works.
What is a SAP calculation?
A SAP calculation is a standard energy assessment used to show that a dwelling meets the energy efficiency requirements of Building Regulations Part L. It looks at the building fabric, heating, hot water, ventilation and any renewables to predict energy use, carbon emissions and fuel bills.
When is a SAP calculation required?
A SAP calculation is normally required for all new build dwellings, many conversions and some extensions where there are significant changes to the thermal envelope or glazing. If Building Control have mentioned Part L, SAP or energy performance, it is very likely that a SAP report will be needed.
What is the difference between As Designed and As Built SAP?
An As Designed SAP calculation is produced from drawings and specifications before work starts, to show that the proposed design will comply. An As Built SAP uses the final specification, air test result and photographic evidence once the dwelling is completed, and is used to lodge the EPC.
Who is allowed to complete a SAP calculation?
SAP Calculations for Building Regulations and EPCs must be completed by an accredited On Construction Domestic Energy Assessor (OCDEA) using approved software. As an Elmhurst accredited OCDEA we are qualified and insured to provide SAP reports for Building Control and EPC lodgements.
What drawings and information do you need for a SAP calculation?
We normally need floor plans, elevations, sections, a site plan with orientation, construction build-ups for walls, floors and roofs, a window and door schedule, and the heating and hot water specification. For more detail, our service pages for new builds, extensions and conversions each list what is required.
How long does a SAP calculation take?
Most single dwellings are turned around within two to three working days once we have all of the information we need. Larger developments, complex conversions or projects with multiple alternative specifications may take a little longer, but we will always give realistic timescales when you book.
What happens if my SAP calculation fails?
If the first SAP result does not meet the Building Regulations targets we will explain why and provide clear recommendations. These might include changes to insulation levels, glazing, heating systems, controls or renewables. We work with you and your designer until a compliant solution is found.
New Build SAP Questions
Questions relating specifically to SAP Calculations for new build houses and flats.
Do all new build homes need a SAP calculation?
Yes. Almost all new build dwellings require SAP Calculations at design stage and As Built stage to satisfy Building Regulations Part L and to create the EPC. This applies to individual self-build homes, small developments and larger schemes.
Do I need an air-tightness test for a new build?
Most new build homes require an air-tightness test under Part L. The test result is used in the As Built SAP calculation and has a big impact on the final rating. In some very small developments an air test may not be mandatory, but using a measured result almost always produces a better outcome than the default.
What is a BREL report and do I need one?
A BREL report (Building Regulations England Part L report) summarises the key design and As Built information for a new dwelling. Building Control often ask for it alongside the SAP reports. Where required, we issue the BREL report as part of the As Built SAP stage.
Can you handle multiple plots and house types on a development?
Yes. We regularly work on small developments with repeated house types and mixed layouts. We will usually create SAP models for each unique type and then apply them across similar plots where appropriate, keeping costs down while still meeting Building Control requirements.
Extension SAP Questions
These FAQs focus on extensions, the glazing rule and when SAP is needed for existing dwellings.
Do all extensions need a SAP calculation?
No. Many extensions are signed off using simple elemental checks in Part L. A SAP calculation is more likely to be required where the extension has a large amount of glazing, roof lanterns, or where Building Control want to see a whole-dwelling heat loss check.
What is the 25 percent glazing rule?
The 25 percent glazing rule is a guideline in Part L which says that, as a starting point, the total area of windows, doors and roof lights in an extension should not be more than 25 percent of the extension floor area, plus the area of any existing openings that are being closed off.
Can I still build my extension if the glazing exceeds 25 percent?
Yes. Many extensions with more than 25 percent glazing can still be approved if a SAP calculation shows that the extra glass is balanced by improvements elsewhere, such as better insulation, higher performance glazing or more efficient heating and hot water systems.
Do roof lanterns and large sliding doors affect SAP?
Yes. Roof lanterns and large glazed doors can significantly increase heat loss and solar gains, which is why Building Control often ask for SAP on highly glazed extensions. We will assess the impact in the SAP model and suggest practical changes if needed.
Conversion SAP Questions
Conversions and change-of-use projects have their own rules in Part L. These FAQs cover the common scenarios.
When does a conversion need a SAP calculation?
A SAP calculation for a conversion is usually required when a building that was not originally a dwelling becomes one, or where you create new self-contained flats. Common examples include barn conversions, commercial to residential projects and houses split into multiple dwellings.
Do garage conversions always need SAP?
Not every garage conversion needs a full SAP calculation. Where a garage becomes a simple habitable room as part of the existing house, compliance may be shown using elemental checks. If the garage becomes a separate dwelling or annexe, or if Building Control request SAP, a conversion SAP calculation is likely to be needed.
What are new thermal elements in a conversion?
New thermal elements are new or significantly upgraded walls, floors and roofs that separate the dwelling from the outside or from unheated spaces. In conversions these elements must normally meet the limiting U-values set out in Part L, and we assess them as part of the SAP conversion calculation.
Do loft conversions require a SAP calculation?
A typical loft conversion forming new rooms within an existing dwelling does not always need its own SAP calculation. Where the loft becomes a separate dwelling, or is part of a wider change of use, SAP may be required. If you are unsure, we can review your plans and confirm this for you.
Overheating and Part O Questions
Part O of the Building Regulations deals with overheating risk in new residential buildings.
What is Part O of the Building Regulations?
Part O is the section of the Building Regulations that deals with overheating in new residential buildings. It requires designers to limit unwanted solar gains and provide adequate means of removing excess heat, particularly in warmer summer conditions.
When is a Part O overheating assessment required?
A Part O overheating assessment is typically required for new build dwellings and certain residential conversions, especially in urban locations or where there are large areas of glazing. Local planning conditions may also explicitly require an overheating assessment.
What is the difference between the simplified method and dynamic simulation?
The simplified method uses set rules on glazing, openings and shading to demonstrate compliance with Part O. Dynamic simulation modelling uses detailed hourly calculations to show internal temperatures over a typical year. Dynamic simulation is more flexible but also more involved and is usually reserved for more complex or higher risk buildings.
Thermal Bridging and Psi-Value Questions
Thermal bridging and psi-values can have a big impact on SAP results, especially in highly insulated homes.
What is thermal bridging?
Thermal bridging occurs where heat flows more easily through junctions in the building fabric, such as at wall–floor junctions, around openings or at eaves. Poorly detailed junctions can increase heat loss and reduce the overall performance of the building.
What is a psi-value calculation?
A psi-value is a measure of the linear heat loss at a junction. Psi-value calculations use specialist software to model junctions in detail. These values can then be used in SAP to reduce the overall Y-value, often improving the SAP result compared to using default figures.
Do I need bespoke psi-values for my project?
Not every project needs bespoke psi-values. Many schemes use approved junction libraries or default values. However, for highly insulated or innovative designs, calculated psi-values can make the difference between a marginal fail and a comfortable pass in SAP.
Pricing and Process Questions
These FAQs explain how our pricing works and what to expect when you instruct us.
How much does a SAP calculation cost?
Our SAP pricing is based on the number of bedrooms, the number of dwellings and the type of project. Extension SAP Calculations use floor area bands. Full details are available on our pricing page, and we are happy to confirm a fixed fee once we have reviewed your plans.
Do you offer fast-track SAP Calculations?
Yes. Where capacity allows we offer a fast-track service for urgent projects, with a guaranteed turnaround for an additional fee. This option is clearly shown during the booking process so you can choose the timescale that suits your build programme.
Do you work nationwide?
Yes. We provide SAP Calculations, conversion SAP, extension SAP and related services for projects across England, Wales and Scotland. All work is completed remotely from your digital drawings and specifications, so there is no need for a site visit in most cases.
How do I book a SAP calculation with you?
You can book online using our booking page, which will guide you through the project type and pricing. Alternatively, you can email your drawings or use our contact form if you prefer us to review the project and confirm the most appropriate option before you commit.
Ready to get your SAP Calculation?
All SAP calculations completed by an Elmhurst-accredited OCDEA and accepted by Building Control nationwide.