London period homes can be wonderfully light, detailed and well located. They can also be draughty, acoustically exposed and subject to heritage controls. Secondary glazing adds a separate internal window behind the original one, so it can improve comfort while leaving the primary window in place. The result depends on the opening, glass, cavity, seals, reveal, installation and wider building.
This guide is for owners of Georgian, Victorian and Edwardian homes across the capital. It explains what a survey should establish, which performance figures are useful for planning, and where local permissions or lease conditions can change the answer. It is guidance for an initial conversation, not a specification or guarantee.
Why London period windows need a property-specific approach
Original sash and casement windows were designed for a different climate, energy market and pattern of use. Slim timber sections, opening joints, worn putty, loose beads and deep reveals can all influence how much heat or sound reaches a room. A flat on a busy road may need a different design from a quiet terrace, even when both have similar-looking sash windows.
The first question is not “which product is best?” but “where is the weak point?” A survey should look at the primary window, wall-to-frame junction, vents, shutters, curtains, room use and noise profile. Secondary glazing can address sound and heat transmitted through a suitable window, but it cannot remove noise entering through a party wall, roof, floor, door or open vent.
What secondary glazing may change
The additional inner pane creates a second layer and an air cavity. In a suitable, well-sealed installation this may:
- reduce draughts around the window;
- make the room-side glazing feel warmer;
- improve acoustic separation from some traffic, rail or neighbourhood noise; and
- retain the original external window and its historic sightlines.
Source comparisons sometimes describe up to 70–80% less perceived noise, or reductions in the region of 40–45 dB and, for certain high-performing specifications, 45–50 dB. These figures are indicative only. They are not universal results: frequency, measurement position, existing window, glass mass, cavity, perimeter seals, installation quality and flanking paths all matter.
Thermal comparisons may describe around 60% less heat loss through the window or a change from an indicative 4.8–5.0 U-value to approximately 1.8–2.2 with a suitable secondary build-up. These are window-level, specification-dependent examples. They do not predict a whole-house bill, EPC band or indoor temperature.
For acoustic detail, see the acoustic-glass guide. For the wider heat and building-fabric context, compare meeting modern thermal standards in a period home.
Choosing a system for a London opening
The opening style should follow how you use the room, how often the primary window needs access and how much depth is available.
| Priority or opening | Possible approach | Points to check |
|---|---|---|
| Tall sliding sash | Vertical sliding secondary unit | Meeting-rail alignment, balance, track depth and cleaning access |
| Casement or inward-opening primary window | Hinged or removable inner unit | Hinge swing, handles, restrictors, curtains and escape access |
| Window rarely opened | Fixed or lift-out unit | Safe removal, ventilation strategy and access to the original pane |
| Busy road or rail line | Reveal-fixed acoustic specification | 100mm+ cavity may be considered; seals, glass and flanking paths remain critical |
| Shutters or deep architraves | Bespoke design after measurement | Historic fabric, shutter operation, sightlines and fixing positions |
There is no responsible “one size fits all” recommendation. A large cavity may help acoustics but use valuable room space. A heavy acoustic pane may improve some frequencies but require suitable hardware. A sliding unit may preserve access but have more moving seals than a fixed frame.
Planning, Article 4 and listed buildings
Because secondary glazing is normally installed internally, planning permission is generally not required for many ordinary homes. That is a starting point, not a blanket exemption. An Article 4 Direction can change permitted-development rights, and a local policy, lease, landlord condition or building-management rule may impose additional requirements.
For a listed building, Listed Building Consent is case-specific. Internal reveals, shutters, panelling, original glass and fixing methods can form part of the significance of the building. Ask the relevant conservation or planning authority about the actual drawings, frame position, finish and fixing method before work begins. The listed-building guide explains the decision points, but it cannot determine consent for an individual address.
In Kensington and Chelsea, a borough comparison can provide useful context on traffic, heritage and specification choices: read the Kensington and Chelsea guide. Local guidance and the status of the individual building still take priority.
What a London survey should cover
A useful survey should record more than window width and height. Ask the installer to document:
- the primary frame material, condition, opening method and existing glass;
- reveal depth, squareness, sill projection, architrave and available cavity;
- shutters, blinds, curtains, radiators, handles and furniture clearances;
- likely sound sources and whether noise may flank through the building;
- ventilation, escape and cleaning requirements between the two layers;
- listing, conservation-area, Article 4, leasehold and landlord considerations; and
- repairs or draught-proofing needed before the secondary frame is fitted.
The survey should lead to a written specification stating glass type and thickness, frame profile and finish, cavity, opening arrangement, seals, fixing method, access plan, making good, VAT and exclusions. A headline “soundproof” or “energy saving” claim is not enough to compare quotations.
Installation without losing the character of the room
Secondary glazing is usually fitted from inside, which may reduce external disruption, but it is not disruption-free. The room needs protection, the reveal must be accessible, and repairs or redecoration may be needed. A careful installer should align inner sightlines with the existing window where practical, avoid unnecessary interference with historic fabric and leave the primary window operable or maintainable as agreed.
The original window remains part of the weather and ventilation strategy. Do not seal over a defect, block a designed vent or assume an inner frame will cure rot. If primary timber is decayed or water is entering at the sill, deal with that before finalising the glazing design.
Cost planning for London
For an early comparison only, the live 2026 UK cost guide discusses broad ranges for standard, acoustic and complex work. London quotations can vary with access, parking, floor level, reveal preparation, glass, finish, number of openings, repairs and making good. Use an itemised survey-based quote rather than applying a per-window headline figure to the whole property.
The cheaper-looking option may omit acoustic glass, finishing, repairs, access equipment or VAT. Ask suppliers to price the same opening schedule and specification so the comparison is meaningful.
A sensible decision framework
Secondary glazing may be worth investigating when:
- the original windows contribute to the character of the home;
- replacement would be visually, technically or legally unsuitable;
- the window is the main route for unwanted sound or cold;
- the reveal can accommodate a properly sealed inner frame; and
- access, ventilation and heritage requirements can be designed together.
It may be the wrong first intervention where the primary frame has serious decay, the main noise path is elsewhere, there is not enough clearance, or the household cannot safely operate two layers. A measured survey can identify those limits before a quotation is accepted.
Frequently asked questions
Is secondary glazing suitable for a London listed home?
It can be suitable in some listed homes because the original window remains in place, but Listed Building Consent is case-specific. The authority may consider fixing positions, reveals, shutters, glass, sightlines and reversibility. Check the actual proposal before ordering.
Will it reduce London traffic noise by 80%?
Some project and product comparisons use up to 70–80% as an indicative perceived reduction. It is not a guarantee. A survey should consider frequency, seals, cavity, glazing, existing window and sound travelling through walls, doors, roofs and vents.
What gap should I ask for?
A gap around 100–150mm is often considered for acoustic work, but the correct dimension depends on the reveal, frame, shutters, ventilation and room use. More space is not automatically better if the unit cannot be sealed or operated properly.
Is planning permission generally needed?
It is generally not required for many internal installations, but Article 4 Directions, listed status, conservation policy, leases and local decisions can change the position. Ask the relevant authority before work begins.
How do I start?
Gather photographs of each opening, note the rooms affected and list your priorities: warmth, traffic noise, neighbour noise, access or preservation. Then request a measured survey and itemised quotation. Contact SecondaryDoubleGlazing for a property-specific discussion.
- * *
SecondaryDoubleGlazing helps owners approach period-window upgrades with measured expectations. The right outcome is the one that fits the building, room and rules that apply to it.