Typically, If you'll be selling or renting it, then you need an EPC and to get an EPC, you need a SAP. The EPC is a report produced from SAP, It's not something separate.

Most Building Control's / Building Standards agencies require an EPC before they'll sign off on completion.

From:

http://www.scotland.gov.uk/Topics/Built-Environment/Building/Building-standards/techbooks/sectsixprg

"Following completion of the proposed building, the Energy Performance Certificate which is to be submitted to the verifier with the completion certificate, should be produced using approved software which implements the current edition and version of the relevant calculation methodology. This ensures that building owners are presented with the most current and relevant information practicable."

The energy assessment (SAP) if correctly entered gives you an idea of the performance of the property, the SAP worksheet can help you to ascertain losses, gains and allows you to compare options - it's not *just* a bureacratic hindrance (It's not a design tool, but that's often how it's used).

SAP2012 allows more flexibility in how things are entered, but SAP2009 is still the current for Scotland for another year or so.