Building Control will usually only ask for SAP calculations if the area of total new glazing in your extension or conservatory exceeds 25% of the extension floor area. NB. 'Glazing' in Building Regulations Part L1B means any openings (doors, windows, rooflights, lanterns, etc).
To show compliance we produce 2 sets of SAP calculations, the NOTIONAL vs the PROPOSED extension. The 'Notional' SAP calculation includes glazing at the 25% maximum area allowed under Part L1B, whereas the 'Proposed' SAP calculation reflects the actual amount of glazing in the proposed extension.
Both sets of calculations include the existing parts of the dwelling, not just the extension, to give a 'whole building' approach to compliance - this has it's advantages, as it makes it possible to include any improvements made to the existing building, to help offset some of the CO2 emissions.
In order to comply the CO2 emissions of the 'Proposed' calculation can no be worse than the CO2 emissions '‘Notional’ extension calculation. It is often the case that a building passes without any extra requirements for the new thermal elements, due to the extra solar gain from the extra glazing, or improvements to the existing parts (e.g. new glazing, extra loft insulation, new boiler).
* Building Regulations Part L is concerned with Conservation of Fuel & Power (or Standard 6.1 of Scottish Building Standards in Scotland). Part L1B is specifically for existing dwellings, i.e. conversions and extensions.