What is the SAP?

It is a calculation of the energy required to heat a house and supply normal uses of electricity for heating, lighting and cooking. It also calculates the CO2 output that the house would have.
The key figures are:

the TER - Target Emmission Rate for CO2 production. This is calculated from the dimmensions of the property.

the DER - Dwelling Emmission Rate, calculated from the rest of the detailed information about the materials and equipment used in the property.
The DER MUST be equal toor less than the TER for the calculation to be deemed a pass.

When should I have the SAP done?

As soon as possible in the design stage. The calculation is required at the first building regulations review. This is immeadiatly after planning has been approved.
Since the calculation can determine wall and floor dimensions, it should be done very early in the planning process.