PART L · Energy performance
Can an extension have large areas of glazing
An extension can have substantial glazing, but the energy design needs to justify it. Under the common approach in the current dwelling guidance, the allowance considers 25% of the extension's floor area plus relevant existing openings covered by the extension. That is not an absolute legal ban on larger windows.
What does the allowance mean
The calculation concerns the combined area of relevant windows, roof windows, rooflights and doors. It is not a separate allowance for each type of opening.
Ask the designer to show the calculation and identify which existing openings are included. A glazed roof and wide rear doors should be assessed together.
Can I go above it
The guidance provides alternative approaches for demonstrating acceptable performance. An appropriate calculation can assess a design with a greater opening area.
The chosen method and assumptions need to suit the project. Do not assume that adding an arbitrary amount of insulation elsewhere automatically balances every glazing proposal.
What information is needed
Use the actual opening sizes and relevant thermal performance information. Details at frames, roof junctions and the existing house also matter.
For example, an early quotation may describe a glazing product generically, while the final doors have a different thermal performance. Review that change before relying on the calculation.
What about overheating
Energy compliance is not a promise of summer comfort. Consider orientation, shading and practical ventilation alongside heat-loss calculations. Part O's specific new-residential scope should not be confused with the wider need to design a comfortable extension.
What should happen before ordering
Agree the compliance approach, complete the necessary assessment and coordinate the glazing with structure and ventilation.
Check the guidance edition applicable to the project. The published 2026 Part L guidance has forthcoming commencement arrangements and must not be mixed indiscriminately with the current standard.