Secondary Glazing for Sash Windows: A Homeowner's Guide
Secondary glazing is a second, separate window fitted on the inside of your existing sashes. It cuts draughts and street noise, and it leaves the original timber and glass untouched. For most period homes in West and Central London, it is the most sensible way to make sash windows comfortable without losing what makes them special.
This guide covers how it works, what it costs, when you need consent, which designs suit sashes, and the mistakes that spoil the finished look. It also answers the question many readers ask alongside it: how sash windows compare with casements.
TL;DR
Secondary glazing adds an inner pane with a wide air gap, so you keep your original sashes and still gain real thermal and acoustic improvement.
Typical professional costs run from roughly £200 to £600 per window depending on the system, so a Victorian terrace with 8 to 10 sashes often lands between £3,000 and £5,000.
It usually needs no planning permission or listed building consent, but check with your borough first if you are listed or in a conservation area.
Sightline alignment, air gap and ventilation decide whether the result looks high-end and performs well.
Sashes suit Georgian and Victorian homes; casements suit later houses. Neither needs to be replaced if the existing windows are sound.
Table of Contents
What secondary glazing does for a sash window
A sash window is a beautiful piece of joinery, but it is rarely airtight. Timber moves, cords stretch, and gaps open around the meeting rails and the sides. Single glass loses heat quickly, and in a busy part of London it lets traffic noise straight in.
Secondary glazing tackles all of this by adding a new window inside the reveal. Historic England’s guidance on secondary glazing sets out the principles, risks and methods for upgrading the thermal performance of older windows this way. The gap between the old and new glass is the key. It acts as a buffer against heat loss and sound.
The figures quoted by installers are encouraging. Bristol Retrofit reports heat loss reductions of 60 to 70 per cent and up to 54dB of noise reduction with high-specification acoustic glass. Treat these as best-case numbers: your result depends on the glass, the seals and the condition of the original sash behind it.
There is a bonus that people often miss. The inner unit also shelters the original sashes from some of the weather and wear, which can help the old timber last longer.
Why not simply double glaze the original sashes?
It is possible to fit slim-profile double glazed units into original sash frames. The trouble is weight and appearance. Heavier glass can upset the balance of the cords and weights, and in a listed or heritage setting, losing historic glass can be unwelcome.
Historic England’s advice on modifying historic windows notes that planning permission may also be needed where permitted development rights have been removed through an Article 4 Direction, which is particularly relevant to homes in conservation areas. Secondary glazing avoids most of this because the original window is not altered.
So the choice often comes down to this:
Secondary glazing | Slim double glazing in original sashes | Full replacement sashes | |
Original glass and frames kept | Yes | Frames kept, glass lost | No |
Consent likely to be needed | Usually not | Often, in listed or protected homes | Often |
Air gap | Large (around 100 to 150mm in a well-designed system) | Small | Small |
Disruption | Often installed in hours, no scaffold | Moderate | Highest |
Best for | Noise and draught reduction in period homes | Where glass is already modern | Windows beyond repair |
The air gap figure comes from Bristol Retrofit. Another guide, from WindowCost, puts the typical gap at 50 to 100mm, so the exact figure depends on your reveal depth. The principle is the same: wider is better, within what your window opening allows.
Planning permission and listed building consent
This is where secondary glazing earns much of its popularity in Fulham, Chelsea, Kensington and Chiswick, where many streets sit in conservation areas and many houses are listed.
Because the system fits inside and does not change the original window, it generally needs neither planning permission nor listed building consent. Axiom Eco Homes makes the same point for listed buildings. The picture is not entirely uniform, though. A local authority guidance note from Lancaster City Council says listed building consent is required to replace or alter the appearance of windows in a listed building, and it covers works that affect historic fabric. Fixing into decorative linings or shutters may count.
Our advice is simple:
Find out whether your home is listed or within a conservation area. Your borough’s planning pages will tell you.
If it is listed, speak to the conservation officer at your borough, for example Hammersmith and Fulham or Kensington and Chelsea, before you order anything.
Ask your installer how the frame is fixed, and whether any historic shutters or linings are affected.
If you want to understand the consent process in more detail, our guides on securing listed building consent for a UK refurbishment and avoiding prosecution by getting listed building consent explain what officers look for.
Choosing the right type of unit
Sash windows are tall, often with shaped heads or deep reveals, and you probably use them daily. That affects which design will suit.
Fixed panels
The simplest and least expensive option. The inner pane is fixed in place and sealed. It performs very well acoustically, but you cannot open it, so it suits windows you never need to reach or clean from the inside.
Sliding and lift-out systems
These are the usual choice for sashes. Horizontal sliders or vertical sliders let you open the window for ventilation or cleaning. Lift-out units can be taken out in spring and summer.
Hinged units
Hinged inner windows open like a small casement. They are good where you need full access to the original sash, for example to paint it, but they cost more.
As a guide, Bristol Retrofit breaks costs down as £200 to £300 for fixed panels, £250 to £600 for sliding or lift-out systems and £350 to £900 for hinged units. Prices vary by region and supplier, so use these as a planning range rather than a quote.
What does it cost?
For a typical Victorian terrace with 8 to 10 sash windows, the same source suggests £3,000 to £5,000 for a professional whole-house installation. Another consumer cost guide, MyJobQuote, puts the average at around £275 to £400 per window and about £3,000 for an average UK home in 2026. The two ranges overlap, which gives you a sensible budget zone.
What moves the price:
Window size and shape. Arched, curved or very tall sashes need bespoke frames.
Glass specification. Acoustic laminated glass costs more than standard float glass.
Frame finish. Powder-coating in a colour matched to your woodwork is usually standard on better systems.
Access. Upper floors and awkward reveals take longer.
Number of windows. Per-window prices tend to fall as the order grows.
Compare this with replacement. The same research puts bespoke double glazed replacement sashes at £800 to £1,500 per window, which is why secondary glazing so often wins on value as well as on conservation grounds.
Getting the details right
Two homes can have the same product and very different results. These are the details that matter.
Align the sightlines
The inner frame’s horizontal bars should line up with your sash’s meeting rail. When they do, the unit almost disappears. When they do not, it looks like an afterthought. Modern slim aluminium frames can be powder-coated in any RAL colour, so ask to see a sample against your own woodwork. If an installer does not talk about alignment, keep looking.
Protect the air gap
A wide gap is what delivers the best acoustic and thermal results. Units fitted tight to the old frame lose that buffer, and they lose much of the sound reduction with it.
Manage ventilation and condensation
A sealed inner window creates a new, separate pocket of air. If moisture gets trapped, you can see condensation and, over time, damage to timber. Make sure the original sashes can still be opened, or include discreet ventilation. Our guide on why ventilation matters in a UK home explains how to get the balance right in a sealed-up house.
Draught-proof the sashes first
Secondary glazing works best on a sound window. Historic Environment Scotland describes draught proofing as the most effective single measure and advises keeping windows in good repair. In practice that means repairing rotten sections, re-hanging sashes and fitting brush seals before the inner unit goes in.
Noise: the case for central London
If the main reason you are looking at this is traffic, buses or neighbours, secondary glazing is the one that makes a difference. The wide gap and a second pane of different thickness work well against low-frequency noise. If your noise problem also comes through walls and floors, glazing is only part of the answer, and our piece on the biggest mistake when soundproofing a London flat explains why. For terraces, see stopping flanking noise under Part E.
Energy efficiency and EPC ratings
Windows are one of the weak points in an older home. One landlord-focused guide from Soundproof My Window says single-glazed units can account for up to 25 per cent of a property’s heat loss. If you let a property in London, upgrading sashes without replacing them is a practical route to a better EPC band. For the bigger picture, read our guide to improving home energy efficiency in 2026.
Sash or casement: which suits your home?
Many readers arrive here wondering whether to keep sashes or switch to casements. The short answer is that the style should follow the house.
Sash windows | Casement windows | |
How they open | Panels slide up and down in the frame | Hinged at the side, swing outwards |
Ventilation | Often only part of the opening is available at a time | The whole opening can be used |
Typical homes | Georgian and Victorian houses | Later semis, cottages, new builds |
Draughts | Gaps tend to appear over time, unless maintained | Compression seals can be tighter |
Character | Strong period feature | Neutral, varied |
If your house was built with sashes, keep them. Swapping to casements changes the facade, can attract objections in a conservation area and removes a feature buyers value. Repair, draught-proof, and add secondary glazing. If you already have casements, secondary glazing can still help, and hinged or sliding inner units work well behind them.
Our perspective
We have worked on period homes across Fulham, Chelsea, Kensington, Chiswick, Hammersmith and Notting Hill since 2006, and the same pattern comes up. Owners come to us for a refurbishment, and the windows are the item they are least sure about. They want the house warmer and quieter, and they do not want to lose the sashes that sold them the house in the first place.
Our view is that the window decision should be made at the start of a project, not at the end. Secondary glazing is usually the least disruptive upgrade, but it works best when the surrounding work is planned with it in mind: the plastering, decorating, joinery repairs and ventilation. If the sashes are repaired and re-hung by the same team that plans the redecoration, nobody has to wait for anybody else.
We run projects in-house under director oversight, with itemised quotes after a site survey, so you know what is being done and what it costs before work begins. For a discerning homeowner, that is the difference between a window upgrade that looks considered and one that looks added on. Read more on our approach in period property renovation that preserves charm and adds modern comfort.
— Mateja
Planning a window upgrade as part of a wider project
If you are already thinking about redecorating, a new kitchen or bathroom, or a full refit, it makes sense to sort the windows at the same time. Our full property refurbishment service covers carpentry, decorating, electrical and plumbing work under one project team, with a fixed, itemised quote after a site survey. Get in touch and we will visit, look at the condition of your sashes and talk through the best sequence for the work.
A simple plan for your sash windows
Check status. Is the property listed or in a conservation area? Ask your borough if unsure.
Assess the sashes. Note rot, broken cords, loose joints and gaps.
Repair and draught-proof the original windows before adding anything.
Choose the system. Fixed for windows you never open, sliding or lift-out for everyday windows, hinged where you need access.
Ask about alignment and gap. Request sightline details and the proposed air gap in writing.
Plan ventilation so condensation does not form between the panes.
Get itemised quotes and compare them like for like on glass, frame finish and fixing method.
FAQ
Can original sash windows be double glazed?
Yes, with slim-profile units, but it is often a compromise. The thicker glass can upset the balance of the sash, and in a listed building the loss of historic glass can be refused. Secondary glazing gives you a larger air gap and leaves the original untouched.
Does secondary glazing need planning permission?
Usually not, because it does not change the original window. If your home is listed, or sits in a conservation area with an Article 4 Direction, check with your borough’s planning team before you order.
Will secondary glazing make my sash windows look bulky?
Not if it is well designed. Slim aluminium frames in a matched colour, with the bars aligned to your meeting rail, are barely noticeable from inside. Poor alignment is what makes units look obvious.
Is secondary glazing better than replacing sash windows?
For most period homes, it is the better value and the more conservation-friendly choice. It costs less per window than bespoke replacements and keeps your original joinery. Replacement only makes sense when the sashes are beyond repair.
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