The draft SAP 2009 contains a number of changes that will affect designers, builders and developers. The most significant changes likely to affect work on site are the need to insulate and seal party walls and the need to pay serious attention to non-repeating thermal bridges.

However, there are also a host of other changes that taken together will have a significant impact. We will not know the full extent until Part L 2010 is released, but the changes in SAP 2009 will in themselves have a considerable impact.

There are essentially two types of change within SAP 2009. First, those aimed to improve the accuracy of the SAP methodology. Second, changes to make SAP more flexible, enabling a variety of new and existing technologies to be combined within a given dwelling.

The overall effect will be an increase in the predicted energy consumption of dwellings for heating. There will also be an increase in energy use in dwellings with air-conditioning as the cooling load is also now considered. The amount of CO2 produced per unit of energy consumed will also increase substantially in many cases, due to a new national assessment methodology. The CO2 emissions from electricity will, for example, increase by 40% between SAP 2005 and SAP 2009.

The key Building Regulations compliance criterion is that the dwelling emission rate (DER) for the proposed dwelling must be less or equal to the target emission rate (TER). The DER and TER are based on carbon emissions per square metre of floor area for heating, hot water and lighting. Many of the changes in SAP 2009 will affect both the DER and TER. This means that housing developers may not feel the full impact of these increases in emissions. It will, however, be harder to achieve a zero-carbon home.