In reality, the addition of an extension is likely to change the energy efficiency characteristics of the dwelling and, in order to ensure that you get the design that you want and that it will comply with the Building Regulations, the SAP Assessment for the extension should ideally be completed as soon as the architect or contractor has finalised the proposed build specification.
This way, should the SAP Assessment result in initial compliance failure it will be relatively inexpensive to make amendments to specifications e.g. insulation thickness or material amendments, prior to commencement of any work on the extension.