Can I double glaze a listed building?

What decides whether your windows can take new glass?

Three things, and they are different for every window. The first is the depth of the rebate, the recess in the sash that holds the glass, because any new glass has to sit within it without the putty or beading changing how the window looks. The second is the size of the glazing bars, since thinner bars may not be strong enough to carry heavier glass and widening them changes the character of the window, which is usually what an officer will question first. The third is the condition of the sashes themselves, and whether any historic glass survives, because original glass that is still in place is something worth keeping in its own right.

These are measured, not guessed. We record the rebate depth, the glazing bar sections and the condition of each sash window by window, so you know which windows can take new glass, which cannot, and why.

Fitting very slim vacuum glazing into original sashes

Vacuum-insulated glass is a recent development, and the slimmest versions are now thin enough to fit many original rebates that would never have taken a conventional double-glazed unit. Where the sashes are sound and the rebates allow it, this means you can improve the thermal performance of a window while keeping the original sashes, glazing bars and box frame, which is the outcome conservation officers are most comfortable with.

Several manufacturers now make vacuum glass of this kind, and they differ in thickness, weight and appearance, so the right choice depends on the window. We will tell you which options suit your sashes, without recommending any particular supplier. Consent is still required, and the application will need to show that the new glass sits within the existing rebates without altering the sight lines.

Slimline double glazing in new like-for-like sashes

Where the original sashes cannot take new glass, or have already been replaced with later sashes of no particular interest, new sashes can be made like for like, matching the original profiles and glazing bar sections, with rebates designed to take a slimline double-glazed unit. The original box frames, which are usually the most significant part of the window, stay in place.

Slimline units have become something of a heritage standard for this kind of work, and when the sashes are made carefully they are very difficult to tell apart from the originals from the street. The detail that matters most is the width of the glazing bars, because a deeper unit often needs a slightly wider bar, and that change is what an officer will look at most closely.

What conservation officers look for

Officers are assessing whether the change affects the character of the building, so they look at how the window will read once the work is done: the sight lines, the width and profile of the glazing bars, the reflection and colour of the glass, and whether original fabric is being kept or lost. Section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 sets the test, and carrying out work that affects the character of a listed building without consent is a criminal offence under section 9 in England and Wales.

An application that shows each window’s condition, explains why the chosen route suits it, and keeps as much original fabric as possible is far more likely to succeed than one that proposes the same specification for every window in the building.

What about secondary glazing?

Secondary glazing is a second, separate pane fitted on the inside of the existing window. It is reversible, and Historic England advises that reversible secondary glazing does not generally need listed building consent, which is why it is often suggested first. In our experience, though, it changes how a window looks and works from inside the room, it can sit awkwardly against shutters and panelling, and it rarely does justice to a fine period interior.

We recommend it only where the other routes are genuinely closed, for example where the original sashes and glass are of such significance that nothing about them can be altered, or where consent for any other option has been refused. In those cases it can be the right answer, and we will say so, but it is rarely where we would start.

The options compared

The table below sets out the four routes, from the one that keeps the most original fabric to the one that keeps the least.

Primary sources

• Historic England, Modifying Historic Windows as Part of Retrofitting Energy Saving Measures
• Historic England, Traditional Windows: their care, repair and upgrading (2017)
• Planning (Listed Buildings and Conservation Areas) Act 1990, section 7
• Planning (Listed Buildings and Conservation Areas) Act 1990, section 9