Secondary Glazing for Listed Buildings

In many London boroughs, slim reversible secondary glazing to an internal reveal does not require consent at all. Where a borough does require it — and Westminster and Kensington & Chelsea often do — applications are routinely approved when the submission demonstrates reversibility, slim sightlines and a sympathetic finish.

We provide the technical drawings, product specification and reversibility statement your conservation officer will ask for, at no charge, as part of the survey.

The secondary frame is mounted onto a timber subframe that is itself fixed into existing plaster or lining, never through mouldings, architraves or the sash box. Removing the system leaves only filled screw holes in modern lining material.

Where reveals are irregular — as they almost always are in Georgian properties — every unit is surveyed and manufactured to the measured opening rather than trimmed on site.

Sightlines are the detail conservation officers examine first. We use slim 20-25mm aluminium sections, aligned with the horizontal meeting rail of the primary sash so the secondary frame visually disappears behind the original glazing bars. Finishes are powder-coated to match existing joinery paint, including traditional heritage whites and off-whites.

Secondary glazing is internal, reversible and does not alter the external appearance of the building, which is why it is the intervention most often accepted on listed windows where replacement or double glazing would be refused outright. That does not mean consent is never needed. Listed building consent is a judgement about impact on special interest, and internal work can still engage it — particularly on Grade I and II* buildings, or where shutters, panelling and original joinery are affected.

Our approach is to assume consent may be required and prepare for it. Every heritage specification is issued with elevation and section drawings, a written statement of reversibility, and a fixing schedule showing that no fastening lands in historic fabric. Where the local authority wants a conversation, we liaise with the conservation officer directly rather than leaving the homeowner to interpret an objection.

The details that attract objection are almost always visual: a frame that cuts across a glazing bar, a colour that reads as modern, or a system that hides original shutters. We match sightlines to the primary window so the secondary frame sits behind an existing bar or margin wherever the geometry allows, and we powder-coat frames to a colour taken from the existing joinery rather than a standard white.

Where shutters are present the secondary frame is set within the shutter box so the shutters still close, which is frequently the deciding factor in a heritage case. On rarely-opened windows we lead with lift-out panels because the removable panel is the easiest specification to defend as reversible.

Heritage windows often have shallow reveals and irregular geometry, so the cavity available is smaller than in a purpose-built opening. We are straightforward about the consequence: a 60mm cavity performs less well at low frequencies than a 150mm one, and no glass specification fully compensates for that. Realistic expectations for a well-executed heritage installation run from the high 30s to the mid 40s dB, rising to up to 54dB in suitable configurations where the reveal allows a deep gap and 10.8mm laminate.

These installations are delivered by our heritage and listed building glazing team, and Heritage-tier builds carry the 25-year warranty rather than the 10-year standard.