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Your extension has too much glazing. Here’s how to fix it.
Published by Low Carbon Box | Sustainable Building Consultants
You’ve spent months working on your extension design. The architect has drawn it exactly how you imagined — big bifold doors, a full-width roof lantern, floor-to-ceiling glazing on the south wall. Then someone mentions that it might not pass building regulations.
This is one of the most common compliance headaches we deal with at Low Carbon Box. The good news: an overglazed extension does not have to mean a redesign. But it does need to be handled properly.
Here’s what the rules actually say, and what the process looks like to get your design approved.
What is the 25% glazing rule for extensions?
Under Part L of the Building Regulations 2021 (Conservation of Fuel and Power), extensions to existing homes are subject to energy efficiency requirements. The default rule is that the total glazed area — windows, doors, rooflights and roof lanterns combined — should not exceed 25% of the extension floor area.
So for a 20m² extension, the allowable glazing is 5m². A set of bifold doors plus a roof lantern can easily exceed that.
Important — blocked openings count towards your allowance. Any existing windows or doors that are removed or blocked up as a direct result of the extension works can be added to the 25% allowance. So if your extension removes an existing French door set measuring 3.68m², your total allowable glazing is not just 25% of the new floor area — it is 25% plus 3.68m². On a 20m² extension that takes the allowance from 5m² to 8.68m², which can make a meaningful difference to what you can fit in.
Does this apply to my project?
| Project type | Does Part L apply? |
|---|---|
| Extension to an existing home | Yes — glazing limit applies |
| Conversion of existing space (e.g. garage) | Yes — same rules |
| Conservatory with separate heating controls and thermal separation | Usually exempt — but check carefully |
| New-build dwelling | Different rules apply (whole-dwelling SAP) |
| Commercial extension | Part L2 applies instead |
Conservatory or extension? This is a common source of confusion. A conservatory is only exempt from Part L if it has a thermally separated door between it and the house, its own independently controlled heating, and more than 75% of its roof area is translucent. Most modern glazed extensions — especially those that open fully into the living space — do not meet this definition. If in doubt, treat it as an extension.
Do bifold doors count? What about rooflights?
Yes — all of them count.
- Bifold and sliding doors: counted as glazing, regardless of how many panels are opaque framing
- Roof lanterns and rooflights: counted as glazing
- Fixed glazed panels: counted
- Opaque walls, even if slim-framed: not counted
This catches a lot of people out. A modest-looking rear extension with a 3m set of bifolds and a roof lantern can easily be sitting at 60–70% glazing before anyone has done the maths.
What happens if my extension has too much glazing?
You have three options under Part L 2021:
Option 1 — Redesign to stay within the allowance. Reduce the glazed area until it meets the threshold. Remember that the full allowance is 25% of the new extension floor area plus the area of any existing openings removed as part of the works — so before assuming a redesign is needed, it’s worth calculating what your actual allowance is. If a set of French doors (3.68m²) is being blocked up to form the new party wall, that area is added back in. Simple in theory, and sometimes the right answer, but often not what the client or architect has in mind.
Option 2 — Area-weighted U-value approach. Improve the thermal performance of the glazing (lower U-values) so that the overall heat loss is no worse than a compliant design. This works in some cases but has limits — very high glazing percentages are difficult to offset with U-values alone.
Option 3 — Full SAP calculation (whole-dwelling method). This is the route set out in sections 10.7 and 10.10 of Approved Document L 2021. It uses SAP 10 software to model the whole dwelling — the existing house plus the proposed extension — and compares its carbon emissions against a reference version of the same extension built to the standard 25% limit. If the proposed design performs as well as or better than the reference design, it passes.
For heavily glazed extensions, Option 3 is usually the most effective route. Here’s why.
Why the SAP calculation works even with lots of glazing
More glass means more heat loss in winter — but it also means more solar gain. On a south or south-west facing extension, that free solar energy significantly reduces heating demand. The SAP calculation accounts for this. A well-oriented extension with good-quality glazing can offset its additional heat loss through solar gains, and actually end up with lower carbon emissions than the reference compliant design.
We’ve seen this on real projects. One recent extension we assessed in Bedfordshire had 73.5% glazing — nearly three times the standard allowance. The SAP calculation showed that solar gains increased by 17.75% compared to the reference design, which more than compensated for the additional heat loss. The extension passed Part L.
This is why the SAP route exists. It’s not a loophole — it’s the regulation recognising that a simple percentage rule can’t account for everything a building actually does.
Typical solutions when the SAP calculation needs help
In the majority of cases, the increased solar gains from a south or south-west facing glazed extension are enough on their own to demonstrate a pass. We run the numbers, the carbon emissions come in below the baseline, and that’s the end of it.
Where the extension faces north, or the solar gains aren’t quite sufficient to close the gap, there are other levers we can look at. SAP models the whole dwelling, so improvements anywhere in the building can contribute to the overall result. Common options include:
- Boiler replacement — upgrading an old gas boiler to a modern high-efficiency condensing boiler can reduce carbon emissions across the whole dwelling
- Additional loft insulation — improving the insulation to the existing house brings down heat loss from the roof and contributes to the overall calculation
- Improved insulation to the extension — specifying better wall, floor or roof insulation than the minimum required can offset some of the additional glazing heat loss
- Better-performing rooflights or glazing — specifying units with a lower U-value, a lower g-value (solar factor) or a better combination of the two can tip the balance
- Other fabric improvements — replacing single-glazed windows in the existing house, or upgrading ground floor insulation where accessible
The aim is always to find the simplest fix first. A boiler replacement the client was planning anyway, or an upgrade from a U-value of 1.8 to 1.4 on the rooflights, is often all it takes. We work through the options with you before committing to a particular specification.
The process: what Low Carbon Box does
Getting a heavily glazed extension approved involves two distinct pieces of work.
Step 1 — Glazing measurement check
Before any calculations can be done, we need to confirm the actual glazing areas from the drawings. We produce a measurement summary that logs every glazed opening — its dimensions, orientation, type (window, door, rooflight), and whether it replaces an existing opening. This document forms the input data for the SAP calculation and is useful for building control sign-off.
Step 2 — SAP assessment
We model the whole dwelling in SAP 10. This involves:
- Modelling the existing house fabric using SAP 2009 Appendix S for pre-1990 construction where records are unavailable
- Adding the proposed extension with the actual glazing specification
- Running the same model again with the reference 25% glazing allowance as the baseline
- Comparing the Dwelling Emission Rates (DER) of both scenarios
If the proposed DER is equal to or lower than the baseline DER, the assessment passes. We produce a written report that can be submitted to building control as evidence of Part L compliance.
What U-values does the extension need to achieve?
The SAP route doesn’t give you a free pass on fabric performance. The extension elements still need to meet minimum standards. Typical requirements under Part L 2021 are:
| Element | Maximum U-value (W/m².K) |
|---|---|
| External walls | 0.18 |
| Ground floor | 0.18 |
| Roof (pitched/flat) | 0.15 |
| Windows and doors | 1.40 |
| Rooflights / roof lanterns | 2.20 |
The better your glazing specification — particularly the U-value and solar factor (g-value) — the more room you have on area. A triple-glazed unit with a U-value of 0.8 W/m².K performs significantly better than a standard double-glazed unit at 1.4. Your architect or window supplier can advise on available specifications.
Our record
Low Carbon Box has a 100% pass rate on overglazed extension SAP assessments. Every project we have assessed using the whole-dwelling SAP method has achieved compliance without requiring a redesign.
This isn’t luck — it’s because we assess the design early, identify any risks before the calculation is run, and work with the architect and client to ensure the glazing specification is right before we start.
How to get started
If your extension design has more than 25% glazing and you need to demonstrate Part L compliance, the process is straightforward:
- Send us your drawings (floor plans, elevations, window schedule if available)
- We carry out the glazing measurement check and confirm the glazing ratio
- We run the SAP assessment and produce the compliance report
- You submit the report to building control as part of your application
Turnaround is typically 5–10 working days from receipt of confirmed information.
Find out more about our overglazed extension SAP service →
Frequently asked questions
How much glazing can I have in an extension?
The default allowance is 25% of the new extension floor area, plus the area of any existing windows or doors that are removed or blocked up as a direct result of the extension works. If the extension removes a set of French doors, that area is added back into your allowance. If you exceed the total allowance, a full SAP calculation can be used to demonstrate compliance by another route.
Do bifold doors count towards the glazing limit?
Yes. All transparent or translucent elements count — bifold doors, sliding doors, fixed glazed panels, windows, roof lanterns and rooflights.
Can I have a fully glazed extension?
In principle, yes — but you would need a SAP calculation to demonstrate compliance. The viability of a fully glazed design depends on the orientation, the quality of the glazing specified, and the thermal performance of the rest of the house. South-facing fully glazed extensions are generally more achievable than north-facing ones.
Do I need a SAP calculation for my extension?
Only if the glazed area exceeds the allowance (25% of the new floor area plus any openings blocked up). If your design is within that limit and the individual elements meet the required U-values, no SAP calculation is needed.
What is Part L of the Building Regulations?
Part L covers the conservation of fuel and power. For extensions to existing homes, it sets out minimum standards for insulation, glazing limits, heating systems and air tightness. Approved Document L Volume 1 (ADL1) is the technical document that sets out how to comply.
Will building control accept a SAP report as evidence of compliance?
Yes. A SAP assessment produced by an accredited On-Construction Domestic Energy Assessor (OCDEA) is the recognised method for demonstrating compliance with the whole-dwelling route under Part L 2021.
Low Carbon Box are accredited On-Construction Domestic Energy Assessors (OCDEA) providing SAP calculations and overglazed extension assessments for homeowners, architects and contractors across the UK. Assessor reference EES/011387.
For more information or to discuss your project: info@lowcarbonbox.co.uk | 01858 322011
