When Do House Extensions Need SAP Calculations?

One of the questions we are most frequently asked is ‘when do house extensions need SAP calculations?’  It is a source of great confusion to home owners and builders.  Does any large extension need a SAP calculation?  No.  It depends on the amount of glazing.  The glazing area of an extension should be no more than a quarter of the floor area plus the amount of glazing that was taken out.  If your new extension has more glazing than this a SAP calculation is required.

Notional Extension

Part L1B building regulations allow a ‘more flexible approach’ for a highly glazed extension.   This involves doing a SAP calculation on the whole house with an extension attached to it which is built exactly the way Building Regulations describe, with exact U values and an exact amount of glazing.  This is called the Notional Extension.  The SAP calculation needs to be done again for the house and the extension as they will actually be built.  The CO2 from the house and the extension that will actually be built needs to be no higher than the CO2 from the house and the Notional Extension.

Using high solar gain glass and extra insulation
Why do this?  It’s so that instead of having to rebuild the whole house in order to pass the SAP test you only have to show that the CO2 emissions are no worse than building regulations require them to be for a house of that age with an extension.  If it isn’t, and the large amount of glazing causes it to fail, it is possible to compensate for this in other ways.  Glass could be used in the extension with a high solar gain, so that the heating effect of the sun in summer would be greater than the heat lost in winter, an extra roll of insulation could be added to the loft, or the boiler could be replaced.  Only minor alterations are needed for the house to pass the SAP test.

For more detailed help and information on extensions and SAP calculations please phone our help line.