Energy Efficient Extensions London 2025
How to build a sustainable extension. Part L requirements, insulation standards, heat pumps, and reducing your running costs.
Quick Answer
Part L 2021 requires extensions to be 31% more energy efficient than previous standards. Expect £3,000-£8,000 additional cost for compliance, but long-term savings on energy bills and improved comfort.
31% more efficient
Part L improvement
0.26 W/m²K
Wall U-value max
£3,000-8,000
Additional cost
Check your specific property constraints
Free Property CheckBuilding regulations have become significantly more demanding for energy efficiency. The 2021 Part L update means extensions must now be 31% more efficient than before. While this adds upfront costs, it reduces long-term running costs and improves comfort.
Part L 2021 Requirements
| Element | Maximum U-value | Typical Solution |
|---|---|---|
| External walls | 0.26 W/m²K | 100mm cavity insulation |
| Flat roof | 0.18 W/m²K | 150mm PIR insulation |
| Pitched roof | 0.16 W/m²K | 200mm mineral wool |
| Floor | 0.18 W/m²K | 100mm underfloor insulation |
| Windows | 1.4 W/m²K | Double glazed, argon filled |
| Doors | 1.4 W/m²K | Composite or insulated |
| Rooflights | 1.7 W/m²K | Double glazed, low-e coating |
Get your extension cost including Part L
Our AI calculator includes the additional costs of meeting 2025 energy efficiency standards. See your full project estimate.
Part O: Overheating
New for 2022, Part O addresses overheating risk—particularly relevant for extensions with large south-facing glazing. This often requires:
Solar Control Glazing
G-value limits on windows to reduce solar gain. South and west facing glazing most affected. Additional cost: £200-£500 per large window.
Openable Windows
Free area for ventilation must be calculated. May need larger openings than you would choose aesthetically.
External Shading
Brise soleil, deep overhangs, or external blinds may be required. Can add £2,000-£5,000 depending on specification.
Wondering what your extension will cost with Part L compliance?
Get a free instant estimate →Heating Options for Extensions
Extend Existing System
Add radiators connected to current boiler. Cheapest option if boiler has spare capacity.
£500 - £1,500
Underfloor Heating
Ideal for open plan extensions. Can be wet (from boiler) or electric. Better with heat pumps.
£1,500 - £3,500
Air Source Heat Pump
Whole-house solution. Most efficient with good insulation. Government grants available.
£8,000 - £15,000
Infrared Panels
Electric option for well-insulated spaces. Low installation cost, higher running cost.
£300 - £800
Improving Your EPC Rating
An extension is a good opportunity to improve your whole home's EPC. Consider:
- Loft insulation top-up: Increase to 300mm for quick wins (£300-£500)
- Upgrade existing windows: While scaffolding is up (£300-£600 per window)
- Smart heating controls: Required for new boiler installs anyway (£200-£400)
- LED lighting throughout: Extension and existing house (minimal cost)
- Solar PV: Consider while roof is accessible (£5,000-£8,000 after grants)
Free AI Assistant
Ask anything about energy efficient extension costs
Frequently Asked Questions
Frequently Asked Questions
Do I have to meet Part L if it's just a small extension?
Yes, all extensions regardless of size must comply with current Part L standards. There's no minimum threshold.
Can I have large bi-fold doors and still comply with Part L?
Yes, but glazing area may be limited to meet energy targets. Your designer will need to balance glazing with insulation elsewhere. Part O overheating may also restrict south-facing glass.
Is it worth exceeding minimum energy efficiency standards?
Often yes, especially for insulation where marginal cost is low. Better than minimum specifications reduce energy bills and improve comfort for decades.
What is Part O overheating?
Part O addresses overheating risk in new buildings and extensions, particularly relevant for extensions with large south-facing glazing. It may require solar control glazing, openable windows, or external shading.
Ready to start?
Plan your energy efficient extension
Get a cost estimate including Part L compliance and energy efficiency upgrades. Free instant assessment.
Or email hello@mayfairstudio.co.uk · 07405 920944