Why Does My Extension Need a SAP Test?

An extension needs a SAP test if the glazing is more than a quarter of the floor area.  This includes doors, windows and rooflights.  If any glazing is removed from the house in order to create the extension this is also added to the amount of glazing allowed.  If the extension has less than this amount of glazing it doesn’t need a SAP test.

You can build a highly glazed extension onto your house if you do something to compensate for the increased CO2.  This could be a new gas boiler, fitting the house with double glazing or putting more insulation in the loft.  The CO2  from the whole house has to be no higher than it would be if you had built the extension exactly according to the Part L1B Building Regulations.  The SAP test is needed to prove this.  The SAP assessor will do two SAP calculations. The first is the notional calculation which shows the house with the extension the way it would be in a text book with the walls, the floor and the roof at the exact U values they have to be and the glazing exactly one quarter of the floor area plus any windows that were removed.  The second calculation is the proposed calculation which shows the house with the extension the way it will actually be built.  If the CO2 from the house with the proposed extension is lower than the CO2 from the house with the notional extension it has passed the SAP test.