London · Q2 2026 · Listed-Building Advice
Secondary Glazing London
Up front — Apex doesn't fit secondary glazing
We're a replacement-glazing installer. We fit aluminium, uPVC, and heritage sash windows under FENSA Reg 40373. We do not currently fit secondary glazing in-house. This page exists because we get asked about it constantly — and for the right type of property, secondary glazing is genuinely the better answer than full replacement. Where it isn't, we'll happily install heritage uPVC or timber-effect alu sash that gets through conservation officer scrutiny.
For secondary glazing fitting in London, we recommend Granada Secondary Glazing, Storm Windows, Mitchell & Dickinson, or Selectaglaze. All four are mature specialists with London track records. The rest of this page is what we'd tell you at a free survey.
What secondary glazing is
Secondary glazing is a thin aluminium frame fitted on the room side of an existing window, with its own pane of glass set 50–150mm in front of the original. The original window stays untouched — frame, glass, sash, ironmongery, all of it. The secondary system attaches to the window reveal, not to the original frame. Two panes plus a trapped air gap deliver near-double-glazed thermal and acoustic performance, without removing the heritage element.
| Component | Detail |
|---|---|
| Frame | Slim aluminium, typically 20–35mm sightline, white / bronze / RAL powder coat |
| Glass | 4–6mm float standard; 6.4mm laminated for acoustic spec; toughened where required |
| Air gap | 50–150mm typical (200mm+ for premium acoustic spec) |
| Opening type | Vertical sliding (sash-match), horizontal sliding, lift-out, hinged casement |
| Fixing | Sub-frame to reveal, no contact with original window — fully reversible |
When secondary glazing is the right answer
Three clear scenarios. Outside these, full replacement is usually the better long-term route.
| Scenario | Why secondary works |
|---|---|
| Listed building (Grade I, II*, II) | LBC for full replacement is unlikely or refused. Secondary glazing is reversible and usually permitted without formal LBC application — check with the conservation officer. |
| Conservation area, front elevation | Local authority requires retention of original windows on street-facing elevations. Secondary preserves the streetscape, replacement does not. |
| Acoustic priority on flight path / A-road | 10–15dB on top of the existing single glazing reaches 38–45dB Rw — equivalent or better than standard double glazing. Wider gap + laminated glass can exceed 50dB. |
| Tenant / leasehold without LL consent | Reversible install means lease terms restricting structural alteration usually still permit it. Confirm with your lease. |
| Phased upgrade where budget caps replacement | £400–£900 per window vs £1,800–£3,500 for sash replacement. Buys time and energy savings until full replacement is affordable. |
Acoustic and thermal performance
Real numbers. Acoustic ratings are Rw (weighted reduction index, EN ISO 717-1). Thermal ratings are whole-window Uw (W/m²K) — lower is better.
| Configuration | Acoustic Rw | Thermal Uw |
|---|---|---|
| Single-glazed Victorian sash (existing) | 25–28 dB | ~5.0 W/m²K |
| + Secondary 4mm float, 100mm gap | 38–42 dB | 2.5 W/m²K |
| + Secondary 6.4mm laminated, 150mm gap | 44–48 dB | 2.0 W/m²K |
| + Secondary 10mm laminated acoustic, 200mm gap | 50–54 dB | 1.9 W/m²K |
| (Reference) Standard double-glazed casement | 30–32 dB | 1.4 W/m²K |
| (Reference) Triple-glazed Passivhaus | 38–43 dB | 0.84 W/m²K |
UK secondary glazing systems
The four specialists we'd point you to in London. None of them are us — but we've worked alongside all of them on mixed listed-building projects.
| System | Best for | Notes |
|---|---|---|
| Granada Secondary Glazing | UK market leader, full opening-style range | Strong listed-building track record, BBA-approved, full London coverage. |
| Storm Windows | Bespoke timber-effect frames | Hardwood and aluminium options, conservation-officer friendly. |
| Mitchell & Dickinson | Period properties, magnetic-fix | Magnetic, removable secondary for very strict listed contexts where any fixing is restricted. |
| Selectaglaze | Acoustic / commercial spec | Heritage England panel supplier, used on cathedrals, theatres and high-acoustic-spec retrofits. |
Cost comparison — secondary vs replacement
London Q2 2026, fitted, per window. Replacement prices are for like-for-like sash window installation; secondary prices are typical specialist quotes.
| Option | Low | High | Notes |
|---|---|---|---|
| Secondary glazing, basic (4mm float) | £400 | £650 | Per window, fitted, no LBC needed. |
| Secondary glazing, acoustic (laminated) | £600 | £900 | +£150–£300 over basic for laminated glass. |
| Heritage uPVC sash replacement | £1,800 | £2,800 | FENSA-certified, double glazed, conservation-friendly profile. |
| Timber sliding sash replacement | £2,400 | £3,500+ | Listed-building approved on most consents; needs LBC. |
| Aluminium heritage casement | £1,400 | £2,400 | Steel-effect alu, slim sightline, modern thermal performance. |
Frequently asked
What is secondary glazing?
Secondary glazing is a thin internal aluminium frame fitted on the room side of an existing window, with its own pane of glass set 50–150mm in front of the original. The original window stays untouched. The two panes plus the trapped air gap deliver near-double-glazed thermal and acoustic performance — without removing the original sash or casement. It's the standard upgrade route for listed buildings and strict conservation areas where replacement isn't permitted.
When should I use secondary glazing instead of replacement?
Secondary glazing is the right answer in three scenarios: (1) listed buildings where Listed Building Consent for full replacement is unlikely or refused, (2) conservation areas where the local authority requires retention of the original window, and (3) acoustic upgrades on roads, flight paths or railway lines where the priority is noise reduction over a 'modern' look. For everything else — non-listed properties without conservation restrictions — replacement double or triple glazing is more thermally efficient, easier to operate, and often the same cost over a 20-year horizon.
How much does secondary glazing cost in London?
Secondary glazing costs £400–£900 per window fitted in London 2026, depending on size, opening type (vertical sliding, horizontal sliding, lift-out, hinged) and frame finish. Compare with £1,800–£3,500 for a like-for-like sash replacement. Acoustic-spec laminated glass adds £150–£300 per window. Prices include site survey, frame, glass, fitting and removal — no FENSA certification needed because the original window stays.
How much noise does secondary glazing block?
Typical reduction is 10–15dB on top of the existing single-glazed window's performance. Single glazing alone typically blocks 25–28dB; with secondary glazing added, total reduction reaches 38–45dB Rw — equivalent or better than standard double glazing. With wider air gaps (150–200mm) and laminated 6.4mm acoustic glass, total reduction can hit 50–54dB. For Heathrow flight paths or A-road frontages, that is genuinely transformative — the difference between a clearly-audible plane and a faint hum.
Does secondary glazing improve thermal performance?
Yes — measurably. A single-glazed Victorian sash typically has a window U-value of around 5.0 W/m²K (very poor). Adding secondary glazing brings the combined U-value down to 2.0–2.5 W/m²K — close to standard double glazing performance. The trapped air gap is the insulator. Performance improves further with low-e secondary glass (1.8–2.0 W/m²K combined). Building Regs Part L now baselines new windows at 1.4 W/m²K, so secondary glazing is a clear improvement on single but not equivalent to a full A-rated replacement.
Do I need Listed Building Consent for secondary glazing?
Usually no — and that's the appeal. Most local authorities and Historic England consider secondary glazing reversible (the original window is unaltered) and therefore permitted under Listed Building Consent rules without a formal application. However, where the secondary frame is visible from outside through the original glass, or where it covers ornate window furniture, an LBC application may still be needed. Always check with the conservation officer before ordering. For full replacement of a listed window, LBC is essentially always required and often refused.
Is secondary glazing ugly?
Modern systems from Granada, Storm Windows, Mitchell & Dickinson and Selectaglaze are far less obtrusive than 1970s-era secondary glazing. Slim aluminium frames in white, bronze, or RAL-matched powder coat sit close to the original reveal. From the street side they're typically invisible behind the original glass. From inside, you see a thin frame at the perimeter and a slight reflection. It's not invisible — but for listed sash windows it's the only legitimate route to acoustic and thermal gain without losing the original window.
Does Apex Glazing fit secondary glazing?
Apex Glazing is a replacement-glazing specialist (aluminium, uPVC, timber, FENSA Reg 40373). We do not currently fit secondary glazing in-house — for that, we recommend specialists like Granada Secondary Glazing, Storm Windows or Selectaglaze. We're happy to advise on whether secondary glazing or full replacement is the right route for your property at a free survey, including in listed buildings and conservation areas across London. Where replacement is the better answer (most non-listed cases) we install heritage uPVC or timber-effect aluminium sash windows that pass conservation officer scrutiny.
