When extensions need SAP calculations
Building Control will usually only ask for SAP calculations if the total new glazing in your extension or conservatory exceeds 25% of the extension floor area. ‘Glazing’ here means all openings — doors, windows, rooflights, glazed roofs and lanterns.
One useful allowance: the area of any existing openings covered over by the new extension can be added to the 25%, giving you more glazing headroom.
How compliance is shown
We produce two sets of SAP calculations:
- Notional — your extension with glazing capped at the maximum 25% allowed.
- Proposed — your extension as actually designed, with its real glazing area.
The proposed extension complies when its CO₂ emissions are no worse than the notional version. Both calculations include the existing dwelling, not just the extension — a ‘whole building’ approach that works in your favour, because improvements to the existing house (new glazing, extra loft insulation, a new boiler) can offset the extra glazing.
In practice, extensions often pass without any additional measures, thanks to solar gain from the glazing itself or modest improvements elsewhere.
What we need
Existing and proposed plans plus a basic specification. It’s a desktop study — no site visit — and turnaround is fast. Get a quote or call 01202 623236.