The two routes to compliance
An extension can comply elementally, by ensuring each new element meets the relevant standard for new fabric and by not exceeding the allowance for glazed area, or it can comply by calculation, where SAP is used to show the extended dwelling performs no worse than a notional version of itself built to the elemental standards. The elemental route is simpler and needs no modelling. The calculation route exists precisely for the designs the elemental route would block.
The 25% glazing threshold
Part L allows new windows, roof windows and doors in an extension up to 25% of the extension's new floor area, plus the area of any existing openings that the extension covers over or removes. Bifolds across a rear elevation, a glazed link or a lantern roof will often take a modest extension past that. Exceeding it is not a refusal; it means the elemental route no longer applies and the extension has to be shown to comply by calculation.
- Measure the new floor area of the extension
- Add the area of existing openings being lost or covered
- Compare against the total area of new glazing and doors
- Over 25%: calculation route; under: elemental route
What the calculation route lets you trade
Because the test compares your extended dwelling with a notional version of itself, extra glazing can be offset. Common routes are better U-values in the new walls, floor or roof, a better glazing specification, improved junction details, or improvements to the existing dwelling's services that were going to happen anyway. Which combination is cheapest is specific to the house, and it is worth checking before the glazing is reduced on the drawings.
Consequential improvements
Work on larger existing buildings can trigger requirements to improve parts of the building beyond the works themselves. Whether that applies depends on the building and the scope, and it is decided by the building control body. We will flag when your project is in that territory rather than leaving it to be raised at inspection.
