The Building Regulations state that the allowable area for openings is 25% of the floor area (of the extension) plus the area of any openings to the existing property that have been removed or are blocked up. As long as this ratio is complied with no further action need be taken. If this ratio is exceeded the extension is regarded as being “overglazed” and in the majority of cases SAP calculations are required. In this case the whole house is rated with a notional extension that is within the 25% ratio & a CO2 emission figure established. The whole house is then re-assessed with the actual extension. The CO2 emission figure is higher due to the increase in the openings. Improvements are then made to the main building such as improved loft insulation, cavity wall insulation, double glazing etc. until the CO2 emission figure in the second calculation is less than in the first. Part L has then been complied with.
Even if you’ve already built the extension only to find that it doesn’t comply we can still help.
elemental Standards for Extensions
Assuming that the extension is not over glazed the following elemental standards for “U” values can be applied. THESE ELEMENTAL STANDARDS CAN ONLY BE USED FOR EXTENSIONS.THEY CANNOT BE APPLIED TO A NEW BUILD OR WHERE THERE IS A MATERIAL CHANGE OF USE.
Some common constructions for achieving the elemental U values
These are just a few solutions with readily available materials. For different constructions or materials, please contact us.