SAP Calculations for domestic extensions

New Building Regulations for energy efficiency came into force in England and Wales in 2014. The English version contains only minor changes from the 2010 standards, but in Wales the insulation standards are more demanding, and there is a requirement for “consequential improvements”.

For extensions to existing houses, if the glazed area is no more than 25% of the floor area, you only need to show that each wall, floor, etc meets the minimum standard for insulation.

If the glazing is more than 25% of the floor area then you will be asked to provide a SAP calculation for the whole dwelling, to prove that the heat loss from the additional glazing has been compensated for elsewhere.

This requires two calculations: one for the house with the extension as it has been designed, and one for the house with a notional extension that meets the 25% glazing area limit. We then compare the two results and show that the first result is no worse than the second. If that isn’t the case then some changes will have to be made.

Experience shows that, with care, it is usually possible to compensate for extra glazing by improving insulation in the extension walls, roof, etc., or by making improvements to the existing house (e.g. a boiler upgrade). BUT you need to find this out before you start work! You don’t want to complete the extension and then find out that you should have put more insulation in the wall cavity.

Obviously then it is vital to do these calculations before the building work starts.

Because the calculations are for the whole house, not just the extension, it is usually necessary to survey the existing building to obtain the detailed data needed for the calculations.