Essentially two alternate extension SAP calculations are produced; the first is based on the existing property with a conventional extension of the same dimensions as the proposed works and a conventional level of glazing. This â€˜Notionalâ€™ SAP calculation provides a calculated level of annual carbon dioxide emissions which in turn provides a target threshold for the proposed works.
A second extension SAP calculation is then prepared, based on the existing house and the specification of the proposed extension, sun room or conservatory.Â The associated annual carbon dioxide emissions are then compared to the previously calculated target rate.
If the proposed extension carbon dioxide emissions are less than the target threshold, then theÂ works are deemed to comply with Part L1B of the Building Regulations via the Design Flexibility Criterion without any additional requirements. In such instances the alternate extension SAP calculations are prepared with a short report for submission to Building Control.
Conversely, if the proposed extension carbon dioxide emissions are greater than the calculated target threshold, then trade-off improvements will be required to either the extension specification or the existing house as a means offsetting the additional heat loss associated with the extra levels of glazing within the proposed extension above the standard threshold detailed within the Building Regulations.
Examples of commonly recommended improvements include additional depths of insulation to the extension floor or roof, improved glazing specificiations, top up loft insulation or cavity wall insulation to the existing property, or improvements to the existing heating system.
The SAP assessment for a conservatory or extension should ideally be completed as soon as the architect or contractor has finalised the proposed build specification for the extension works, since if the SAP calculations result in an initial compliance failure, it is far easier to recommend subtle amendments to specific insulation thicknesses or material amendments before any work has commenced.