Building-Regs FAQ
What is Part L?
Part L of the Building Regulations covers thermal performance and energy efficiency. New extensions must meet U-value targets for walls, roofs, floors, glazing.
Part L of the Building Regulations covers thermal performance and energy efficiency. New extensions must meet U-value targets for walls, roofs, floors, glazing.
Part L is the section of the Building Regulations covering conservation of fuel and power. For an extension it sets maximum U-values for new elements — 0.18 W/m²K for walls and floors, 0.16 for a pitched roof, 1.4 for windows and doors — and limits the total area of glazing.
The elemental route is the normal one: meet the U-values for each new thermal element, and keep glazing within 25% of the extension's floor area plus the area of any windows or doors covered up by the extension. Meet those and no calculation is needed.
Where glazing exceeds that limit — a large glass rear extension — compliance is shown by calculation instead, demonstrating that the extended dwelling's carbon emissions are no worse than a notional compliant version. That is a SAP-based exercise requiring an assessor.
Existing thermal elements being renovated have less demanding threshold values — 0.30 W/m²K for a wall — and even those can be relaxed where the measure is not technically or functionally feasible, or where the payback exceeds fifteen years. On a solid-wall Victorian house that judgement should be recorded, with a condensation risk analysis to BS 5250.
Heating system efficiency and controls, hot water storage, lighting efficacy, and the limitation of heat loss at junctions — thermal bridging. Bridging is the most commonly underestimated input: using default psi values rather than accredited construction details can cost several SAP points.
Yes. New roof slopes, dormer cheeks and flat roofs are new thermal elements and must meet the new-build U-values. Insulating between and under rafters is the usual build-up, and the depth needed is one reason head height gets tight.
How what is part l? plays out in Islington: London Borough of Islington (Islington Council) maintains 40 conservation areas, the Barnsbury Conservation Area and the Canonbury Conservation Area among them, and an Article 4 direction is in force there, so permitted development cannot be assumed. Anyone dealing with what is part l? at a Islington address is therefore working to a £548 householder application decided in 8 weeks, or a £249 prior approval over 6 weeks where what is part l? still qualifies. are the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| Wall U-value, new element | 0.18 W/m²K |
|---|---|
| Pitched roof | 0.16 W/m²K |
| Window and door | 1.4 W/m²K |
| Rooflight | 2.2 W/m²K |
| Glazing limit | 25% of extension floor area plus displaced openings |
| Existing wall on renovation | 0.30 W/m²K threshold |
Yes, usually only by demonstrating compliance through a whole-dwelling calculation.
Only when renovating them, subject to feasibility and payback.
No. It sets performance standards, not technologies.
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