Secondary glazing is a second pane of glass installed inside your existing window — usually in its own slim frame mounted to the internal reveal. It’s particularly common in listed buildings and conservation areas where replacing original windows isn’t permitted, and in period properties where keeping the original sashes is aesthetically important. This guide covers 2026 UK pricing and when secondary glazing is the right choice vs full replacement.

Typical UK secondary glazing prices in 2026

Per-window installed cost (ex-VAT):

Style	Small window	Medium window	Large / bay
Lift-out / hinged	£280 – £420	£380 – £580	£500 – £800
Horizontal sliding	£350 – £520	£460 – £680	£580 – £900
Vertical sliding (sash-matched)	£420 – £620	£540 – £780	£680 – £1,050
Acoustic specification (upgrade)	+£120 – £180	+£150 – £230	+£200 – £310

For a typical 8-window Victorian terrace in Greater London: secondary glazing whole-house ~£3,200 – £5,500 vs sash replacement at £11,000 – £18,000. Secondary glazing saves 70-75% on capex while still delivering most of the thermal and acoustic benefit.

What secondary glazing does (and doesn’t do)

• Thermal performance: reduces heat loss by ~60% vs single glazing alone. U-value typical 1.8-2.2 W/m²K (vs ~5.0 for single glazing, vs 1.2-1.4 for modern double glazing)
• Acoustic performance: excellent — the airspace between the two panes absorbs sound. Typical 40-50dB reduction, often better than standard double glazing
• Condensation: reduced significantly — the inner pane stays warmer, so interior moisture doesn’t condense on it
• Doesn’t deliver: the u-value of modern sealed-unit double glazing. For maximum efficiency, sealed double or triple is still better. Secondary glazing is a step up from single — not a substitute for sealed DG

When secondary glazing is the right answer

• Listed buildings where replacement windows aren’t permitted
• Conservation areas with Article 4 restrictions on front-elevation changes
• Preserving original Victorian/Georgian sash windows you love aesthetically
• Rented properties where structural changes aren’t allowed but you want to reduce noise / cold
• Noise-focused upgrade (acoustic performance often better than sealed double glazing)
• Tight budget — 70% cheaper than full sash replacement in heritage properties

When full replacement is better

• The original frames are rotten or structurally compromised — fix the frames first
• Maximum thermal performance matters more than preserving original windows
• You’re already doing extensive renovation and the glazing cost difference is marginal
• The existing windows have failed sealed units rather than single glazing — replacement is cleaner than layering

Styles available

• Hinged (side-opening): cheapest, most basic. Opens into the room like a cupboard door
• Lift-out: removable panels. Good for windows accessed rarely (e.g., stairwells). Storage space needed when removed
• Horizontal sliding: panel slides left/right. Useful for wide windows
• Vertical sliding sash-matched: matches original sash action — most aesthetically pleasing in Victorian interiors. Premium option
• Double-glazed panel: the secondary pane itself is double-glazed. Rare and expensive — gives near-triple-glazing performance in heritage properties

Planning and listed-building considerations

Because secondary glazing is internal and doesn’t change the external appearance, it rarely requires planning permission — even on listed buildings. This is its major advantage over full replacement. However, on Grade I or Grade II* listed buildings you may still want listed-building consent for any significant internal change. Check with your local conservation officer before installing.

Frequently asked questions

Is secondary glazing as good as double glazing?

For thermal performance, no — double glazing wins on u-value (1.2-1.4 vs 1.8-2.2 for secondary). For acoustic performance, secondary glazing is often better because of the larger airspace between panes (50-100mm vs 12-16mm). For a noise-priority installation near a busy road or flight path, secondary glazing actually outperforms standard double glazing.

Can I fit secondary glazing to sash windows?

Yes — vertical sliding sash-matched secondary glazing is specifically designed for this. The secondary sash operates similarly to the original, preserving the functional feel. Expect to pay 15-20% more than basic horizontal-sliding secondary glazing for the matched-sash style.

Does secondary glazing look ugly from outside?

No — it’s internal. From outside your property looks identical. The secondary frame sits within the interior reveal, usually matching the window colour or painted white. Heritage installations use slimline anodised aluminium or painted timber to minimise visual impact from inside too.

How long does secondary glazing installation take?

Per window: 30-60 minutes. Whole-house (8 windows): 1 day. No making-good or plastering required because the frame sits within the existing reveal. Minimal disruption compared to full window replacement.

Can I open the original window with secondary glazing fitted?

Yes — that’s the point. Lift-out, hinged and sliding secondary glazing all allow easy access to the original window behind for opening, cleaning, or summer ventilation. You just open the secondary first.

Get four secondary glazing quotes

Secondary glazing isn’t FENSA-registered work (it’s not a replacement window) but competent installation still matters. Findfitter matches with 4 local heritage-glazing specialists — free, no obligation. Specify “secondary glazing” in your enquiry so only the right installers respond.

Sources

• Historic England — window advice for listed buildings
• Energy Saving Trust — secondary glazing thermal performance data
• UK heritage-glazing installer market data 2024-2025