How Much Does Secondary Glazing Cost? Real Per-Window Prices

Supplied and fitted, made-to-measure aluminium secondary glazing in London generally lands somewhere between £300 and £700 per window. The national average sits lower, around £350 a window across the UK, and London and the South East typically run 15 to 25% above that.

That is a wide band for what looks like a simple product, and the spread is not sales margin. Two windows in the same house can be £280 and £900 apart for reasons a homeowner would not think to ask about. Here is what moves the number.

Price by system type

Secondary glazing is not one thing. The frame configuration is the biggest single driver, because it determines the amount of aluminium, the number of moving parts and the labour on site.

Vertical sliders cost more than horizontal ones for a straightforward mechanical reason: the panel has to be counterbalanced. Spiral balances or spring mechanisms are built into the jambs, they have to be specified to the weight of the glass, and a 6.4mm acoustic laminate panel is roughly 60% heavier than 4mm float. They are also the only system that lets you open the original sash and the secondary panel together in the same movement, which is why people pay for them on windows they actually use.

Horizontal sliders are the workhorse of the trade. They are cheaper, they are mechanically simple, and on a wide Victorian bay they suit the proportions. On a tall narrow Georgian sash they look wrong, because the meeting stile of the slider cuts a vertical line across an elevation that has none.

Working it out per square metre

Most of the industry quotes internally by area, then adds fixed costs per opening. Typical rates are roughly £200 per m² for a fixed panel, £300 for lift-out, £350 for hinged and £400 for horizontal or vertical sliding, before glass upgrades.

A common London sash reveal is about 900mm wide by 1,800mm tall. That is 1.62 m². At £400 per m², the frame and standard glass come to around £650, and that is before survey, delivery and fitting are separated out. A tall first-floor Georgian window at 1,000 by 2,100mm is 2.1 m², and the same rate puts it near £840.

This is why "how much per window" is a bad question and "how much for these windows" is a good one. Measure your own reveals before you ring anybody. It takes ten minutes and it tells you immediately whether a quote is in the right postcode.

The glass is a separate decision

Glass is priced by area and specification, and the difference between the cheapest and the most useful is smaller than people assume.

On a 1.6 m² window, moving from 4mm float to 6.4mm acoustic laminate is roughly £110 of glass. That is the single best-value line item on the whole quote if noise is any part of why you are doing this, and it is routinely left off the cheapest option so the headline number looks better. Ask for it to be priced both ways.

The laminate works because the plastic interlayer damps the panel rather than just adding mass, and because two dissimilar thicknesses of glass either side of a wide air gap do not resonate together. The full picture on what actually reduces traffic noise through a sash window is in our article on cutting traffic noise on a busy London street.

Cavity depth: the thing that pulls two ways

The gap between your original sash and the secondary panel is set by where the secondary frame lands in the reveal, and it is a genuine conflict.

For heat, a narrow cavity is better. Once the gap exceeds roughly 20mm, air starts to circulate in it, carrying heat from the warm inner pane to the cold outer one. Beyond that the thermal return flattens off.

For noise, a deep cavity is better, and the gains keep coming up to 200mm and beyond. Specialist acoustic installations aim for 100mm to 200mm between panes.

You cannot have both, and where the frame sits determines which you get. In a shallow London reveal with a plastered return you may only be able to achieve 60mm, which is a meaningful acoustic downgrade and nothing can be done about it after the survey. If noise is the priority, say so at the survey and ask what cavity is achievable, in millimetres, before you sign.

The seven things that push a quote up

1. Out-of-square reveals. Nothing in a 150-year-old house is square. Where the discrepancy is more than a few millimetres, the installer fits a timber sub-frame liner to give the aluminium something true to sit on. Reckon on £80 to £200 a window, and it is worth every penny — a frame packed and mastic-filled into a wonky reveal looks like an afterthought.
2. Working shutters. Original panelled shutters and secondary glazing fight for the same space. Either the secondary frame sits inside the shutter boxes, which shrinks the cavity, or it sits in front of them, which stops the shutters ever being used again. The compromise costs money and sometimes costs the shutters. If yours still work, read our piece on shutters for sash windows before you commit, because closed shutters are a serious thermal performer in their own right.
3. Curved or arched heads. A radiused head means a curved frame section, and that is specialist work. Expect a premium of 50% to 100% over a rectangular opening of the same area.
4. Colour. White polyester powder coat is the standard finish and is included. A RAL colour, a bronze anodised finish or a timber-effect foil typically adds 10 to 20%.
5. Window boards and radiators. A radiator directly under the window, or a deep window board with a moulded nosing, often forces a cill-mounted sub-frame or a raised sill detail. It is not difficult, but it is not standard either.
6. Escape windows. Any window that serves as a means of escape from a bedroom or upper storey has to stay openable, which rules out fixed and some lift-out panels on those openings. Raise it at the survey, and check the position with your installer rather than assuming.
7. Access and floor protection. Ground-floor work off a hallway is quick. A stairwell window over a half landing, with a heavy laminate panel and no room for two men to swing it, is a different day's work.

VAT: the detail nobody mentions

Secondary glazing is charged at the standard 20% rate of VAT. It is not covered by the zero rate for energy-saving materials, which runs to 31 March 2027 and then reverts to 5%. HMRC's guidance lists what qualifies — insulation, heat pumps, solar, heating controls — and then says explicitly that secondary and double glazing and low-emissivity glass are standard-rated.

Draught stripping, on the other hand, does qualify for the zero rate.

What a quote should itemise

Ask for these separated out. A single figure per window makes comparison impossible and hides the decisions.

1. Frame system and configuration, named — vertical slider, two-track horizontal, lift-out.
2. Frame finish and whether it is standard white or a chargeable colour.
3. Glass specification by thickness and type, with the acoustic laminate priced as an option.
4. Cavity depth achieved, in millimetres, window by window.
5. Sub-frame or liner, where one is needed, priced per opening.
6. Removal and making good, including whether the existing window furniture has to come off.
7. Survey and delivery, and whether the survey fee is credited against the order.
8. VAT, shown separately, at 20%.
9. Guarantee period on frame, glass and hardware, which are frequently different.

The honest payback

This is the part most cost guides avoid.

Take that 900 by 1,800mm single-glazed sash. As a whole window it is losing heat at something like 4.8 W/m²K. Add low-emissivity secondary glazing and you are down to roughly 1.8. Over a London heating season, that difference works out at somewhere around 200 to 250 kWh a year for that one window. On mains gas, that is roughly £15 to £25 a year.

Against £550 for the window, the simple payback is measured in decades, not years. Anybody presenting secondary glazing as an energy investment is selling you something.

What it does buy, immediately and noticeably:

• Quiet. This is the real reason people fit it, and it is the one thing sealed units cannot match. The comparison is set out in secondary versus double glazing.
• The metre nearest the window becomes usable. Cold glass pulls radiant heat off you whether or not the air is warm. Raise the inner surface temperature and a sofa under the window stops being the seat nobody chooses.
• Condensation moves off the glass you have to wipe, provided the secondary frame is sealed on the room side and the original sash is left slightly leaky to the outside. Get that the wrong way round and you trap moisture in the cavity.
• Security. A locked secondary panel is a second barrier, and insurers occasionally recognise it.

Where to spend and where not

Do the rooms you sit in, facing the noise. A front bedroom and a front sitting room on a bus route will change your life. A rear box room will not.

Do not glaze over windows you use daily with a system that makes them awkward to open, because within a year you will stop opening them and the ventilation will go with them.

And get the original sashes right first. Secondary glazing works with the primary window, not instead of it. A rattling sash with perished putty leaks sound straight into the cavity and undoes the specification you paid for. Our sash window repair service covers overhaul, draught-proofing and re-puttying across London, and it is a sensible sequence: repair, seal, then glaze if the noise still bothers you. If you want the full menu of options and roughly what each achieves, insulating sash windows sets them side by side.

Get three quotes, all specified to the same cavity depth and glass, and measure your reveals before the first surveyor arrives.