Glossary
Part L: energy efficiency for London extensions and lofts
Conservation of fuel and power. Thermal performance, energy efficiency, and insulation.
Conservation of fuel and power. Thermal performance, energy efficiency, and insulation.
Approved Document L sets the energy standards for building work. For an extension the elemental route applies: maximum U-values of 0.18 W/m²K for walls, 0.15 for a flat or pitched roof at ceiling level, 0.18 for floors and 1.4 for windows and doors, together with limits on the total area of glazing.
For new thermal elements in an extension: external walls 0.18 W/m²K, pitched roof insulated at ceiling level 0.16, pitched roof insulated at rafter level 0.16, flat roof 0.18, floors 0.18, windows and doors 1.4, and rooflights 2.2. Existing elements being upgraded have their own, less demanding, threshold values.
Glazing area in an extension is limited to 25% of the extension's floor area plus the area of any windows or doors covered up by it — a limit that catches large rear glazed extensions and is then dealt with by a whole-dwelling calculation instead.
Where the elemental route fails on glazing area, compliance can be shown by calculation: demonstrating that the extended dwelling's carbon emissions are no worse than they would be for a notional compliant extension. That is a SAP-based exercise and needs an assessor.
Achieving 0.18 W/m²K on a solid brick wall requires around 100 mm of insulation internally or externally, which is often impractical or unwanted. The regulations allow a lower standard where the measure is not technically or functionally feasible or where the payback exceeds 15 years — but that judgement should be recorded, with a condensation risk analysis to BS 5250.
How approved document l (conservation of fuel and power) plays out in Croydon: London Borough of Croydon (Croydon Council) maintains 24 conservation areas, the South Norwood Conservation Area and the Crystal Palace Conservation Area among them, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Anyone dealing with approved document l (conservation of fuel and power) at a Croydon address is therefore working to a £548 householder application decided in 8 weeks, or a £249 prior approval over 6 weeks where approved document l (conservation of fuel and power) still qualifies.
| Wall U-value, new element | 0.18 W/m²K |
|---|---|
| Pitched roof at ceiling level | 0.16 W/m²K |
| Flat roof | 0.18 W/m²K |
| Floor | 0.18 W/m²K |
| Window and door | 1.4 W/m²K |
| Glazing area limit | 25% of extension floor area, plus displaced openings |
Yes, but usually only by demonstrating compliance through a whole-dwelling calculation.
Only where you are renovating them, and even then the feasibility and payback tests apply.
2.2 W/m²K for a new rooflight in an extension.
Pick a slot that suits you. No obligation and no sales pressure, just straight answers on your planning route.
