Overglazed Report

Why Have I Been Told To Get An Overglazed Extension Calculation?

SAP Calculations – for over-glazed extensions & conservatories (Part L1B). Sometimes referred to as Notional vs Proposed SAP calculations.

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).

For example, if your extension is 10m2 then your glazing/opening allowance would be 25% of this area, which is 2.5m2. We can also include any doors/windows which have been removed to accommodate the extension. Example, if you had to remove a glass sliding door and a window which totalled 3m2, then we can add that too the aforementioned 2.5m2, allowing a total glazing area in the extension of 5.5m2

To show the extensions 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 its 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 must be equal to or lower then the CO₂ emissions of the ‘Notional’ extension calculation. Often, the building passes without any requirement for changes but if it fails, changes MUST be made to either the extension or the existing dwelling. This will be discussed if the property fails. 

Why Choose Us For Your Overglazed Calculation?