Secondary Glazing Planning Permission & Listed Building Consent in London
A plain-English guide to exactly what consent you need before installing secondary glazing — whether your home is unlisted, in a conservation area, or a Grade I listed building.
Last reviewed September 2026 · Written by London Secondary Glazing
The short answer
- Planning permission is almost never required. Secondary glazing is internal and does not alter the exterior, so it falls outside planning control — even in a conservation area.
- Listed buildings need listed building consent. This is separate from planning permission. Secondary glazing is usually approved because it is reversible and preserves the original window.
- Historic England endorses secondary glazing as an appropriate way to improve heritage windows without replacing them.
Do you need consent? Quick reference
Find the row that matches your property. When in doubt, a quick call to your borough's conservation team settles it.
| Your property | Planning permission | Listed building consent |
|---|---|---|
| Unlisted home, not in a conservation area | Not required | Not applicable |
| Unlisted home inside a conservation area | Not required | Not applicable |
| Flat in a conservation area (leasehold) | Not required | Check lease for freeholder consent |
| Grade II listed building | Not usually required | Usually required |
| Grade II* or Grade I listed building | Not usually required | Required — consult conservation officer early |
This table is general guidance, not a substitute for advice from your local planning authority. Always confirm your property's status before starting work.
Three terms that decide everything
Listed building
A building on the National Heritage List for England, protected for its architectural or historic interest. Changes need listed building consent. Roughly 92% are Grade II.
Conservation area
A designated area of special character. External changes are restricted, but internal secondary glazing is not controlled — so no permission is needed unless the building is also listed.
Article 4 direction
A local rule that removes some permitted development rights in a specific area. It affects external work; internal secondary glazing remains outside its scope.
Listed building grades and what they mean for glazing
The grade determines how much scrutiny an application receives — not whether secondary glazing is allowed. It is approved across all three grades when specified sympathetically.
Buildings of exceptional interest
Listed building consent is required. Expect close scrutiny; slim, discreet, fully reversible systems and prior discussion with the conservation officer are essential.
Particularly important buildings of more than special interest
Listed building consent is required. Reversibility and sympathetic sightlines are the deciding factors in most approvals.
Buildings of special interest warranting every effort to preserve them
Listed building consent is usually required, but secondary glazing is widely approved because it is internal, reversible and preserves the historic window.
What Historic England says
Historic England — the government's adviser on the historic environment — recognises secondary glazing as one of the most effective ways to improve the thermal and acoustic performance of historic windows while keeping the original glass and joinery in place.
The decisive quality is reversibility. Because a secondary unit is fitted within the existing reveal and can be removed later without harming the historic fabric, it is far more readily accepted than replacing period windows with modern double glazing — which is usually refused on listed and conservation-area properties.
You can confirm your property's status directly on the National Heritage List for England and check conservation-area boundaries on your borough council's planning pages.
Building Regulations and thermal performance
Historic and listed buildings benefit from exemptions under Part L where compliance would unacceptably alter their character. Secondary glazing improves performance without the obligations a full replacement would trigger. Lower U-values mean less heat loss.
| Window configuration | U-value (W/m²K) | Notes |
|---|---|---|
| Single glazing (typical original window) | 4.8 - 5.0 | Baseline period window |
| Single glazing + secondary glazing | 1.8 - 2.0 | Up to ~60% less heat loss |
| Single glazing + low-E secondary glazing | 1.6 - 1.8 | Best retrofit performance |
| Modern replacement double glazing | 1.4 - 1.6 | Usually not permitted on listed windows |
Acoustic performance you can expect
The wide air gap between the original window and the secondary unit is what makes secondary glazing outperform double glazing for noise. A reduction of 10 dB is perceived as roughly halving the loudness.
| Configuration | Noise reduction | In practice |
|---|---|---|
| Original single-glazed sash | ~25 dB | Little protection from traffic or rail |
| Secondary glazing, 100mm air gap | up to 45 dB | Noticeably quieter rooms |
| Secondary glazing, 150-200mm gap + laminate | up to 54 dB | Suitable for flight paths and busy roads |
How to secure listed building consent, step by step
- 1
Confirm your property's status
Search the National Heritage List for England and your borough's conservation area map. This tells you whether you need listed building consent, and which local policies apply.
- 2
Speak to your conservation officer
For listed buildings, an early, informal conversation with the borough conservation officer almost always smooths the process. They will tell you what detailing they expect before you apply.
- 3
Specify a reversible, sympathetic system
Slim aluminium sightlines, matched paint finishes and units set within the existing reveal are what officers look for. Nothing should be fixed into historic joinery in a way that cannot be undone.
- 4
Submit the listed building consent application
Applications go through the Planning Portal or your borough. There is no fee for listed building consent. Include drawings, product specifications and a short heritage statement explaining reversibility.
- 5
Install once consent is granted
With consent in hand, installation is quick and causes no disruption to the external elevation. Keep your consent documentation with the property records.
Frequently asked questions
Not sure what consent your property needs?
We handle listed building and conservation-area work across London every week, and we can advise on consent as part of a free, no-obligation survey.
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