Part L Compliance For Extensions
SAP Calculations for Extensions
Most extensions don't need a SAP calculation. Use our free calculator to check the 25% glazing rule, or find out which compliance route applies to your project.
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Extension Part L Compliance
Quick Reference TL;DR
Do I need SAP for my extension?
Probably not. Most extensions pass using simpler methods. SAP is the last resort, not the default.
What's the 25% glazing rule?
Glazing can be up to 25% of floor area + any openings you cover. Under this? No SAP needed.
I'm over 25% - now what?
Try area-weighted U-values first (£95). Better insulation elsewhere can compensate for extra glass.
When do I actually need SAP?
Only if reference method AND area-weighted both fail. Solar gains often save heavily glazed designs.
What about conservatories?
Thermally separated = often exempt. Open to house = treated as extension.
How do I check my glazing?
Use our free calculator. Enter floor area + openings, get instant pass/fail.
Not sure which route?
Start with the free glazing calculator. It tells you if you're within limits or need further checks.
Do I Need a SAP Calculation for My Extension?
Probably not. Most extensions comply with Part L of the Building Regulations using simpler methods and never need SAP. Building Control only require a SAP calculation when the reference method and area-weighted checks don't work - typically for heavily glazed designs where insulation upgrades can't compensate for heat loss through glass.
Three Ways Your Extension Can Comply With Part L
Building Control doesn't automatically require SAP for extensions. Part L gives you three routes to compliance, and you only move to the next if the simpler method doesn't work.
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1
Reference Method
Glazing ≤25% of floor area (+ covered openings) and U-values meet Part L limits. No calculation needed - Building Control sign off on spec alone.
Free to check yourself
Check your glazing free → -
2
Area-Weighted U-Value
Slightly over on glazing? If your average insulation across all elements beats the notional standard, you pass without full SAP.
£95 - credited toward SAP if needed
How area-weighting works → -
3
Full SAP Calculation
Models the whole house + extension. Solar gains and heating efficiency often approve designs that failed simpler methods.
From £200
When you need full SAP →
Reference Method - The 25% Glazing Rule
The reference method is the simplest route to Part L compliance. As described in section 10.7 of Part L, if your extension meets two conditions, Building Control can approve it without any energy calculation:
How to Avoid a SAP Calculation for Your Extension
- Glazing area must be ≤25% of extension floor area, plus the area of any existing openings you're covering over
- U-values for walls, floor, roof and openings must meet Part L limiting standards
This is how most rear and side extensions are approved. Standard double glazing, reasonable insulation, and modest window sizes typically pass without issue.
The 25% glazing allowance is more generous than people expect and can avoid a SAP Calculation. If you're removing a back door and window to open up into the extension, that area gets added to your allowance. A 20m² extension removing a 2m² door and 1.5m² window would have a glazing allowance of 8.5m² (20 × 0.25 + 3.5).
Check Your Glazing in 60 Seconds
Enter your floor area, existing openings and proposed glazing. Get an instant pass/fail result with a PDF you can share with Building Control.
Extension Glazing Calculator →Area-Weighted U-Value Method
If your glazing exceeds 25%, you move to the area-weighted method. Section 10.9 of Part L shows how we can compare your proposed extension against a notional version with compliant glazing levels.
The calculation averages U-values across all thermal elements (walls, floor, roof, windows, doors, rooflights) weighted by their areas. If your average is no worse than the notional extension, you pass.
How area-weighting can save your design:
- Extra roof insulation compensates for additional glazing
- Triple glazing offsets a larger window area
- Better wall U-values balance out bifold doors
- High-performance rooflights reduce impact of roof lanterns
This method works well for extensions that are slightly over on glazing. Large rear doors with a well-insulated flat roof often pass. Multiple roof lanterns with standard insulation elsewhere may struggle.
Area-Weighted Check - £95
We calculate the area-weighted U-value for your extension and confirm if it passes. If it doesn't, the £95 is credited toward a full SAP calculation.
Request Area-Weighted Check →Full SAP Calculation
When both the reference method and the area-weighted approach don't work, a full SAP calculation models the entire dwelling - existing house plus extension - to demonstrate compliance. This is in Part L under section 10.10.
This is how heavily glazed, architect-led designs get approved. SAP accounts for factors the simpler methods ignore:
- Solar gains: South and west facing glass lets in free heat. Large sliding doors facing the garden often perform better than expected.
- Heating efficiency: A modern boiler or heat pump improves the whole-house calculation.
- Existing house performance: A well-insulated existing property can absorb more glazing in the extension.
- Thermal mass: Solid floors and masonry walls store and release heat, improving overall performance.
Most extensions that reach this stage still pass. North-facing glazing is harder (no solar gains to offset heat loss), but targeted upgrades - triple glazing, extra roof insulation, better airtightness - usually bring results into compliance.
Extension SAP Calculation - From £200
Full SAP modelling for extensions with high glazing. Includes design stage assessment, recommendations if needed, and reports for Building Control.
What's Included in Our Extension SAP Service
- Full SAP model of the existing dwelling plus proposed extension
- Assessment against Part L targets with clear pass/fail result
- Practical recommendations if initial result doesn't pass (ranked by cost-effectiveness)
- Reasonable design stage revisions included
- Reports formatted for Building Control submission
- As-Built SAP available at completion if required
- Email support throughout the project
We're not here to gold-plate your extension. If a cheaper fix gets you over the line, that's what we recommend.
How It Works
Check Your Glazing
Start with our free glazing calculator. If you pass the reference method, you may not need us at all. If you're over, we'll tell you the next step.
Send Your Drawings
Upload plans, elevations, glazing sizes and construction details. We'll confirm whether area-weighted or full SAP applies.
Get Your Result
We complete the calculation and send your report. If changes are needed, you get clear recommendations before Building Control see anything.
Extension Compliance Pricing
Reference Method Check
Free
Use our online calculator to check glazing and U-values against Part L limits.
Open calculator →Area-Weighted Check
£95
Professional calculation of area-weighted U-values. Credited toward SAP if this method doesn't pass.
Request check →Full SAP Calculation
From £200
Complete SAP modelling for extensions with heavy glazing. Design stage + Building Control reports.
View full pricing →Extension SAP - Frequently Asked Questions
Do all extensions need a SAP calculation?
No. Most extensions comply using the reference method (glazing under 25% + standard U-values) and don't need SAP at all. You typically need SAP only when glazing exceeds limits and the area-weighted method can't compensate.
What is the 25% glazing rule?
Part L allows glazing up to 25% of your extension floor area, plus any existing openings you're covering over. Stay within this and meet standard U-values, and Building Control can sign off without SAP.
Can I have more than 25% glazing?
Yes. If you exceed 25%, the area-weighted U-value method checks whether better insulation elsewhere compensates. If that doesn't pass, a full SAP calculation can still approve the design by accounting for solar gains and whole-house performance.
What's the difference between the three methods?
Reference method is the simplest (glazing under 25%, standard U-values, no calculation needed). Area-weighted averages your insulation performance across all elements. Full SAP models the entire dwelling including solar gains and heating efficiency.
How much does an area-weighted check cost?
Our area-weighted U-value check is £95. If it doesn't pass, we credit that toward a full SAP calculation, so you're not paying twice.
How long does an extension SAP calculation take?
Most are completed within 2-4 working days once we have drawings. The area-weighted check is usually same-day.
Do conservatories need a SAP calculation?
If thermally separated from the house (external doors between, independent heating), often exempt. If open to the house or sharing the main heating, it's treated as a normal extension and the same rules apply.
Will Building Control accept your reports?
Yes. All work is completed by Elmhurst accredited assessors using approved software. Our reports 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.