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Over-Glazed Extensions and Part L Compliance
Large areas of glazing are one of the most desirable features in modern extensions. Many people are not aware there are requirements under Building Regulation Part L that set out the maximum allowable glazed area. Planning an extension with more than the 25% allowance, read on.
What architects and homeowners need to know under Part L 2021
Large areas of glazing are one of the most desirable features in modern extensions. Whether it’s a full-width sliding door, panoramic windows, or a roof lantern, natural light transforms living spaces. From a compliance perspective, more glass also means more heat loss and that’s where Over-Glazed SAP Calculations come in.
When does an extension become “over-glazed”?
Building Regulations state that if the total proposed glazed area exceeds 25% of the new floor area, the extension is classed as over-glazed.
Glazing includes:
- Windows and glazed doors
- Roof lights and roof lanterns
- Any new glazed elements being installed as part of the works
Once that 25% threshold is crossed, compliance with Part L 2021 must be demonstrated through a SAP (Standard Assessment Procedure) calculation.
This process confirms that the additional glazing won’t cause excessive energy loss or an increase in CO2 emissions therefore ensures the extension performs efficiently year-round.
Key U-Value Requirements
At the heart of Part L compliance are U-values, which measure how well a building element prevents heat loss (the lower the number, the better).
Typical target values are:
- Walls: 0.18 W/m²K
- Floors: 0.18 W/m²K
- Roofs: 0.15 W/m²K
- Windows and doors: 1.4 W/m²K
- Roof lights: 2.2 W/m²K
Modern construction often outperforms these targets. For example, a well-insulated brick and block cavity wall can achieve U-values as low as 0.12 W/m²K.
Exceeding the minimum standards not only improves your chance of passing the SAP assessment but also reduces long-term energy bills and carbon emissions.
Three Routes to Compliance
Part L allows for three recognised methods of demonstrating compliance:
- Reference Method
- Area-Weighted U-Value Method
- Whole Dwelling Calculation Method
At Energy Report, we prefer to use the Whole Dwelling Method. It offers greater flexibility, particularly for large glazed areas, and allows for compensating measures such as improved insulation elsewhere or the inclusion of renewable technologies like solar PV.
This approach often makes the difference between a failed and a passed assessment without compromising the architectural vision.
What’s Required for a SAP Assessment
No site visit is necessary. The assessment is desk-based, requiring only:
- Plans and elevations of the proposed extension as well as the existing property
- Construction specifications (walls, roofs, floors, glazing, doors)
- Current heating system
If the initial specification doesn’t meet the standards, our assessors will recommend practical adjustments – balancing budget, timeline, and construction constraints to achieve compliance.
Fees and Turnaround
Our fees for an Over-Glazed Extension SAP Calculation start from £280 + VAT for straight forward single-storey or modest glazed extension.
Pricing varies depending on project size, complexity, and whether additional services (such as updated U-value calculations, thermal bridging advice, or re-submissions) are required.
Typical turnaround is 2–3 working days, and we provide clear, actionable recommendations to help your project meet compliance efficiently.
Why Choose Energy Report Limited?
Since 2007, Energy Report Limited has supported architects, developers, and homeowners in achieving full Part L compliance across the South of England. Our team translates complex energy regulations into clear, actionable strategies.
We provide:
- SAP Calculations for over-glazed extensions
- Advice on optimising U-values and insulation
- Early design input to avoid costly redesigns later
- Turnkey compliance packages for Building Control approval
Large glazed extensions can transform a home but only if designed and built with energy efficiency in mind. With professional SAP analysis and expert guidance, it is possible to achieve both design ambition and regulatory compliance.
Need help with your over-glazed extension?
Let’s make compliance straightforward.
Call Energy Report Limited on 01489 883231or visit energyreport.co.uk to request your Over-Glazed SAP Calculation today.
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