Building Regulations, Part L and Part F Explained

Why secondary glazing is usually exempt

The regulations are triggered by replacement of a controlled fitting. Secondary glazing does not replace the window, it supplements it. The original window and its frame remain, which is why installations are not normally notifiable and why there is no FENSA style certificate for secondary glazing. That said, exempt does not mean unconstrained, and a competent installer designs to the same principles.

Part L: conservation of fuel and power

Part L sets standards for thermal performance in new and replacement work. Where secondary glazing is installed as part of a wider project, a conversion or a whole house retrofit, the assessor will want a U value for the combined glazing element. A Low E coated secondary pane in a reasonable cavity brings a single glazed opening down into a range that satisfies most retrofit targets, and on protected buildings there is explicit recognition that historic fabric may justify a different standard.

• Ask for a calculated U value on the quote
• Low E soft coat glass is the usual thermal specification
• Historic buildings have a proportionality allowance

Part F: ventilation

This is the part that matters most in practice. Sealing a leaky window removes ventilation the house was informally relying on, and if nothing replaces it, humidity rises and mould follows. Part F sets background ventilation expectations, and the sensible response is to design ventilation in: trickle vents in the secondary frame, retention of existing background vents, or an appropriate mechanical solution in a wet room. Never block an existing vent and leave nothing in its place.

Part B: means of escape

Where a habitable room above ground floor relies on its window for escape, that route must remain usable. In practice this means specifying an openable secondary unit rather than a fixed or lift out panel, with clear opening dimensions preserved and an operation that works in darkness and under stress. This is one of the few places where we will push back on a customer's preference for the cheapest option.

Part K and safety glazing

Glass in critical locations, low level areas below 800mm, glass beside doors, and glazing in bathrooms, should be safety glass. Toughened and laminated both qualify. Since acoustic laminated glass is already the usual acoustic choice, this requirement is normally satisfied by the specification you would want anyway, but it should be checked on low cills and door side lights.

Where it becomes notifiable

If secondary glazing forms part of a larger project that is itself notifiable, a conversion, an extension, a change of use or a material alteration, then the glazing is assessed within that submission rather than on its own. In those cases give the building control body the specification, U values and ventilation provision at the outset rather than at completion.

What we provide

For every installation we can supply the glass specification, cavity depth, calculated U value and the ventilation provision per opening. That documentation is what an assessor, building control officer or EPC assessor needs, and it is worth keeping with the property paperwork for the next owner.