As MCS says, if you are selling a home before it has been built, you will need to provide information about energy efficiency in a Predicted Energy Assessment (PEA). This is the predicted SAP Rating and Environmental Impact (CO2) rating from the SAP calculations that should have been done at the design stage, to check whether you comply with Building Regulations. PEA's do not have a Recommendation Report like full Energy Performance Certificates.
For dwellings completed on or after 6 April 2008, you will need to give the prospective owner a full EPC and recommendation Report. The EPC can only be produced by an Accredited On Construction Energy Assessor. People who are currently doing SAP calculations to show that they meet Target Emission Rates as part of Building Regulations will not be able to produce the EPC unless they have undergone additional training and joined an accreditation scheme.
An On Construction Energy Assessor should not normally need to visit the property to produce an EPC as the calculations can be made from scale plans and the design specifications. The EPC will be the result of the As Built calculations which is an extension of the SAP calculation needed for Building Control.
Building Control are unable to issue a completion certificate without an EPC and the sale of the property cannot proceed without an EPC.