Energy Efficient Extensions London 2025
How to build a sustainable extension. Part L requirements, insulation standards, heat pumps, and reducing your running costs.
Key Requirements for 2025
- Part L 2021: 31% more energy efficient than previous standards
- Wall U-value: Maximum 0.26 W/m²K (typically needs 100mm+ insulation)
- Additional cost: £3,000-£8,000 to meet new standards vs old
- Get instant cost estimate for your extension (Free)
Building 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 |
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.
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)
Building an Energy Efficient Extension?
Get an instant cost estimate including energy efficiency requirements for your project.
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?
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 standards?
Often yes, especially for insulation where marginal cost is low. Better than minimum specifications reduce energy bills and improve comfort for decades.
What government grants are available?
The Boiler Upgrade Scheme offers £7,500 towards heat pump installation. Check eligibility for your property.