Secondary Glazing Building Regulations

Part L in practice: what assessors accept

Approved Document L1B treats secondary glazing as a controlled fitting only when the whole window assembly is altered; a reversible internal secondary unit is normally assessed as an improvement measure rather than a replacement. In practice, an original single-glazed sash at roughly 4.8 W/m²K improves to about 1.9 W/m²K with a low-emissivity secondary pane, and to about 1.6 W/m²K where a wider cavity and Pilkington K glass are combined. Those figures are sufficient for SAP and RdSAP uplift, which is why the measure is routinely used to lift a rented property over the MEES minimum EPC threshold without touching the façade.

Part F: keeping background ventilation intact

Where the primary window has trickle vents, the secondary unit must not seal them off. The two compliant details are an aligned acoustic-attenuated trickle vent through the secondary frame, or a deliberately ventilated cavity that leaves the existing vent path open. Habitable rooms require 8,000mm² equivalent area and wet rooms 4,000mm²; acoustic vents typically cost 3–5dB of measured performance, so on noise-critical elevations we specify mechanical extract instead and record the decision in the handover pack.

Part B: egress and escape windows

Any opening relied on for escape must retain a clear openable area of at least 0.33m² with a minimum 450mm clear height and width, and a cill no higher than 1,100mm above floor level. On escape openings we specify side-hung or lift-out secondary panels with a single-action release that can be operated without a key, so the escape route stays usable in darkness.

Part K and Part N: impact safety glazing

Critical locations — glazing below 800mm from floor level, and glazing within 300mm of a door edge below 1,500mm — must meet BS EN 12600 Class 2(B)2 or better. Laminated glass satisfies this while also carrying the acoustic and security benefits, so it is our default in low-cill Georgian and Victorian reveals rather than a toughened alternative.

Listed buildings, Article 4 and consent

Building Regulations and planning consent are separate consents. Listed Building Consent is usually required for Grade I and Grade II properties and is normally granted because the installation is reversible and the historic fabric is preserved. In conservation areas and Article 4 zones a short pre-application enquiry removes the risk; our compliance hub carries a borough-by-borough contact directory and a ready-to-send enquiry template.

Documentation issued on completion

• Glass specification sheet with U-value, Rw and C_tr figures per opening
• BS EN 12600 impact classification certificates for critical locations
• Ventilation strategy record demonstrating Part F equivalent areas
• Egress schedule for any opening forming part of the escape route
• Reversibility statement for listed building and conservation area files