office@energycheck.org.uk

The building of an extension to an existing dwelling must comply with Part L1B of the Building Regulations in respect of heat loss, lighting and heating provision.

An energy rating (SAP) can only be carried out on the whole dwelling, not the extension

Compliance of the structural elements is by virtue of maximum U-values (heat loss parameters).

Openings, such as windows, external doors and rooflights should not exceed 25% of the extension floor area (plus the area of redundant existing openings).

PROBLEMS

The commonest problem is that of overglazing. 

There are two approaches to the problem. 

The first is an assessment of the overall heat loss from a notional extension of the same size, built to the maximum U-values allowed, using the glazing rule above, compared to that of the designed extension. This is usually only possible when there is only minor overglazing.

The second approach considers the existing dwelling plus the extension. A SAP calculation is carried out for the dwelling as existing plus a notional extension ... the result is the DER (dwelling emission rate) of carbon dioxide. This is compared against the existing dwelling (including energy reducing enhancements) plus the extension as designed.

It is the proposed energy reducing enhancements to the existing dwelling that counteract effect of overglazing. Past improvements such as cavity wall insulation or more efficient boiler, cannot be taken into account, even if very recent.

The Provision of Thermal Elements as per Table 4

Element	Max. U-value Standard in an Extension
Wall	0.30
Pitched Roof – insulation at ceiling level	0.16
Pitched Roof – insulation at rafter level	0.20
Flat Roof or Roof with Integral Insulation	0.20
Floor	0.22

 

Upgrading Retained Thermal Elements

Element	Max. Improved Value
Cavity Wall	0.55
Other Wall Type	0.35
Floor	0.25
Pitched Roof – insulation at ceiling level	0.16
Pitched Roof – insulation at rafter level	0.20
Flat Roof or Roof with Integral Insulation	0.25