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SAP Calculations for Extensions
Part L compliant SAP Calculations for extensions, completed by Elmhurst accredited assessors. Fast turnaround for clients nationwide.
Suitable for rear, side and wrap-around extensions, including projects exceeding the 25 percent glazing guidance.
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Do I Need a SAP Calculation for My Extension?
Many extensions are signed off under Part L using simple elemental checks, but some projects need a SAP calculation to show that the extension and existing house together still provide reasonable energy performance. This is most common where the design includes large areas of glazing or where there are significant changes to the thermal envelope.
You are more likely to need an extension SAP calculation if your project includes a large rear or side extension with extensive glazing, one or more roof lanterns, or where new walls, floors and roofs are added with a lot of glass. Building Control will often ask for a SAP report when the glazing appears to exceed the 25 percent guidance or when the design falls outside the simple examples in Approved Document L.
If you are not sure whether your extension needs a SAP calculation, send us your drawings and a short description of the work. We will review the plans and confirm what is required for Part L compliance before you commit to any fees.
Understanding the 25 Percent Glazing Rule
The 25 percent glazing rule is a simple way for Building Control to check whether an extension is likely to be reasonable in energy terms without needing a detailed SAP calculation. It is often quoted as a hard limit, but in practice there is more flexibility than many homeowners are told.
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 external openings that are being filled in or reduced as part of the work. This is the basic 25 percent glazing guidance in Building Regulations Part L for existing dwellings.
If your design goes over this level, Building Control do not have to refuse the extension. Instead, they can ask for a SAP calculation to show that the extra glazing is balanced by improvements elsewhere, such as better insulation to new thermal elements, higher performance glass or more efficient heating and hot water systems. In many cases a well designed extension with large glazing can still be approved once these trade offs are shown in the SAP report.
We specialise in explaining the glazing rule in clear terms and using SAP Calculations to justify extensions that fall outside the simple 25 percent guidance.
What Is Included in Our Extension SAP Service
Our SAP service for extensions is built to give quick, practical answers when glazing and insulation questions start to slow a project down. We help you and your architect find a specification that both meets Part L and keeps the design you want.
- SAP extension calculations for rear, side and wrap-around extensions, including projects with large areas of glazing.
- Assessment of new thermal elements such as walls, roofs and floors, with checks against limiting U-values for existing dwellings in Part L.
- Glazing and roof light assessment to understand the impact of large doors, picture windows and roof lanterns.
- Heating and hot water system checks, ensuring that the extended house still has efficient, well controlled services.
- Clear pass or fail summary with targeted recommendations if the first result does not meet the Part L requirements.
- Reasonable design stage revisions included, so you can adjust insulation levels, glazing and heating specification without extra SAP fees each time.
- As-Built SAP for the whole dwelling if required at completion.
- Email support for your architect, builder and Building Control officer throughout the extension project.
Pricing for Extension SAP Calculations
SAP Calculations for extensions are priced per dwelling with fixed, transparent fees. Simpler extensions with modest glazing are at the lower end of the range, while complex layouts and very large glazed areas may require a little more work. We will always confirm the fee before we start.
Standard Extension SAP
From £200 per dwelling for extension SAP calculations where glazing is close to the 25 percent guidance and the layout is straightforward.
High Glazing or Complex Extensions
From £225 to £250 for extensions with very large glazed areas, multiple roof lanterns or more complex layouts that need additional modelling.
As-Built SAP
Optional As-Built SAP for the whole dwelling where an extension Design SAP has been completed.
How the Extension SAP Process Works
The SAP process for extensions is designed to be as simple as possible. We work from your architect drawings and a few key specifications, then explain exactly what needs to change, if anything, for the extension to be acceptable under Part L.
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Send us your extension drawings and glazing details
Upload your floor plans, elevations, sections and a note of the window, door and roof lantern sizes. If you have any construction details or U-values from your architect, send those too. If anything is missing we will let you know what is needed.
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We complete your extension SAP calculation
We build a SAP model that reflects the existing house and the proposed extension. We assess the glazing against the 25 percent guidance and check new walls, floors and roofs against the limiting U-values in Part L. If changes are needed, we provide clear, cost focused recommendations.
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You receive your SAP report for Building Control
Once the extension SAP calculation shows reasonable energy performance, we issue your SAP report and any supporting Part L documentation. These can be sent directly to Building Control so your extension can progress without delay.
Most extension SAP calculations are completed within one to two working days once we have the information we need. If you have a deadline from Building Control, tell us when you book and we will let you know what is realistic.
What We Need for an Extension SAP Calculation
Extension projects often move quickly once planning is agreed. Having the right information ready for your SAP calculation helps avoid delays when Building Control ask for evidence on the glazing rule or insulation levels.
- Floor plans with dimensions and room labels, showing both the existing house and the proposed extension.
- Elevations and sections showing the extension, existing openings and any new windows, doors and roof lights.
- A site plan or simple note of orientation so we can model solar gains correctly.
- Construction details or outline build ups for new walls, floors and roofs, including insulation type and thickness where known.
- A schedule of windows, doors and any roof lanterns with sizes and, if available, U-values and glass specifications.
- Heating and hot water system details, including boiler or heat pump type, emitters and controls.
- Ventilation details, for example intermittent extract fans, dMEV, MEV or MVHR systems if used.
If you do not have full specifications, we can work with your architect or suggest reasonable assumptions and improvements. We will always highlight where assumptions have been made so you can decide which upgrades give the best value against the SAP results.
Extension SAP Frequently Asked Questions
Do all extensions need a SAP calculation?
Not every extension needs a SAP calculation. A SAP calculation is normally required where the extension has a large amount of glazing, where the work creates significant new thermal elements, or where Building Control ask for a SAP report to show that the design is reasonable under Part L. If you are unsure, send us your drawings and we will confirm whether your extension needs a SAP calculation free of charge.
What is the 25 percent glazing rule for extensions?
The 25 percent glazing rule is a simple check in Approved Document L for extensions. As a guide, the total area of windows, doors and roof lights in the extension should not be more than 25 percent of the floor area of the extension, plus the area of any existing openings that are being closed off. If you go above that, Building Control will often ask for a SAP calculation to show that the design still provides reasonable energy performance.
Can I still build my extension if the glazing is over 25 percent?
Yes, many extensions with large areas of glazing can still be approved. A SAP calculation for the extension can demonstrate compliance by improving the insulation to new thermal elements, upgrading glazing, improving air tightness or using more efficient heating and hot water systems. We will advise on the most cost effective changes if your first result does not meet the Part L requirements.
How long does an extension SAP calculation take?
Most extension SAP calculations are completed within one to two working days once we have all of the necessary drawings and specifications. If you have a tight Building Control deadline, let us know when you book and we will confirm what is achievable.
What drawings do I need to provide for an extension SAP?
For an extension SAP calculation we need floor plans, elevations, sections, glazing sizes including any roof lanterns, basic construction details and your heating and hot water specification. If you do not have full U-values, we can work with your architect or suggest reasonable assumptions and improvements.
Will Building Control accept your extension SAP reports?
Yes. All extension SAP calculations are completed by an Elmhurst accredited OCDEA using approved software. Our reports are written for Building Control and are accepted by local authorities and approved inspectors across the UK.
Ready to get your SAP Calculation?
All SAP calculations completed by an Elmhurst-accredited OCDEA and accepted by Building Control nationwide.