A sash window has one or two panels (sashes) that slide vertically within the frame, the traditional style of Victorian and Georgian homes. Modern versions carry slim double-glazed units that keep the period look while meeting today's energy rules. A fitted uPVC sash costs £600 to £1,500 in 2026, while timber heritage sashes cost more again. Price your own set in the sash window calculator.
If you already own sashes, the real questions are whether you can double glaze the ones you have, whether the conservation officer will let you touch them, why one will not stay open, and what a restoration costs against a replacement. This guide works through each in turn.
How does a sash window work?
The classic box sash has two panels that slide past each other on cords and counterweights, or on modern spring balances. You can open the top and bottom together to draw cool air in low and push warm air out high, which is part of why they suit tall period rooms. The trade-off is more mechanism than a casement, which is why a sash costs more to make and fit. Compared with the simpler hinged style covered in our casement windows guide, a sash is the dearer but more authentic choice for an older home.
Cords and weights or spiral balances: what is the difference?
Every sliding sash has to be counterbalanced or it would fall shut, and which system you have decides what a repair costs. A traditional cord and weight hangs a cast iron weight on a cord running over a pulley at the head of the frame and down inside a hollow box built into the jamb. That box is why original sash frames are so deep. The only consumable is the cord, so the system is repairable more or less indefinitely.
A spiral balance replaces all of it with a spring-loaded tube recessed into the side of the frame. No weights, no pulleys, no box, which is why modern uPVC sashes and slim timber replicas can be so much shallower. The balance is a sealed unit: when it weakens and the sash creeps down you swap it rather than repair it. Cheap to service, but it ties you to the manufacturer for matching parts years later.
What are sash horns, and why do they matter?
The small curved projections at the bottom corners of the upper sash are horns. On a Victorian window they are structural: once large plate-glass panes arrived and glazing bars disappeared, the corner joint needed reinforcing, and the horn is the extension of the stile that does it. Georgian sashes, with small panes and heavy bars, usually have none. Horns are the detail a conservation officer looks at first. uPVC and modern timber sashes normally carry mock horns, moulded or applied rather than structural, and some are far more convincing than others. Ask for a sample or a close-up photograph of the exact profile before you order, never a brochure render.
A well-made sash on a Victorian frontage is a thing of beauty, and it sells houses. But it is not a casement, and you pay for the extra engineering. Do not order sashes for a 1980s semi just because they look posh.
Tom Bradley, window installer
Sash window costs in 2026
Material drives the price, and with sashes the choice is usually between uPVC and timber. uPVC sashes give you the look and the energy performance at the lower price. Timber sashes cost more but are often the only option allowed on period and listed homes, where secondary glazing can be the only way to improve a window you cannot replace; timber rewards the spend with a 30 to 60 year life if maintained. These figures are cross-checked against Checkatrade cost data.
| Material | Per window (fitted) | Notes |
|---|---|---|
| uPVC sash | £600–£1,500 | Period look at the lower price |
| Aluminium sash | £950–£2,030 | Less common; slim, durable |
| Timber sash | £1,330–£2,850 | Heritage and conservation homes |
Source: DGCC 2026 dataset, cross-checked against Checkatrade.
Why do published timber sash prices vary so much?
Because they genuinely do, and you should know that before treating anyone's figure as gospel. Take a 600 x 900 mm timber sash, supplied and fitted. Two widely cited UK sources, both current in July 2026, put very different numbers on the same product.
| Source | Timber sash, 600 x 900 mm (fitted) | Same source, uPVC sash |
|---|---|---|
| Checkatrade | £1,400–£1,580 (£1,490 typical) | £600–£720 (500 x 1,000 mm) |
| Federation of Master Builders | £2,450 | £805 |
Sources: Checkatrade timber windows cost guide and FMB window prices, both July 2026.
The FMB figure is roughly 1.6 to 1.8 times the Checkatrade figure for an identical window, and the same pattern holds on uPVC. Neither is wrong: they sample different trades, regions and specifications, and "timber sash" covers everything from a softwood replica to an engineered hardwood heritage unit. Treat any single published price as an indication and let three written quotes settle it. Our sources are set out in the methodology.
Can you double glaze an existing sash window?
It depends This is the question most sash owners arrive with, and the answer is often yes, with conditions. A specialist takes the sashes out, routs the original rebate deeper to accept a slim-profile sealed unit, reglazes and rebalances them, then puts them back in the original box frame. You keep the frame, the pulleys, the horns and the proportions. Checkatrade prices fitting double glazing into existing sash frames at £650 to £1,300 per window as of July 2026.
The conditions catch people out. Historic England accepts that where historic windows survive without historic glass, it may be possible to introduce slim-profile double glazing without harming the significance of a listed building. But it flags the compatibility problem directly: thicker glazing sometimes forces renewal of the window frame itself, at which point you have lost the thing you were trying to keep. If original crown or cylinder glass survives, reglazing destroys it. And on a listed building you need listed building consent before any of this happens, not after.
Be sceptical, too, of a very slim unit quoted with an impressive U-value. The thinner the cavity, the harder the physics, and the edge seal works harder than on a standard unit. Ask what the guarantee actually covers if it mists, which our guide to window condensation explains is the failure mode that matters.
Would secondary glazing be a better answer?
Frequently, and it is under-considered. Secondary glazing adds an independent inner pane on the room side and leaves the original window untouched, at £300 to £900 per window against £400 to £1,000 for replacement double glazing. Historic England notes it would only affect the special interest of a listed building if it damaged the frame, panelling or shutters, a far lower bar than reglazing. On a busy road it also beats double glazing on noise, because the wide air gap is a better sound barrier than a narrow sealed cavity. The honest downsides are another frame to clean behind and two windows to open instead of one.
Do sash windows need consent in a conservation area?
This is where sash windows demand care. If your home is in a conservation area or is listed, you usually cannot simply fit whatever you like. Planners often require timber sashes that match the original proportions, glazing bars and sightlines, and you may need consent before any work starts. Always check with your local planning authority first. Heritage replacements can also attract help: see the grants guide for the schemes that may apply, though most are income-based rather than tied to property age.
The distinction worth holding onto is between the two designations, because homeowners routinely conflate them.
- A conservation area controls the appearance of the street. Councils commonly withdraw permitted development rights for window replacement using an Article 4 direction, in which case you need planning permission for a change that would otherwise be free. Whether that applies depends on your specific area, so the only safe answer is to ask. Our conservation area rules guide and conservation area cost page cover the detail.
- A listed building is protected in itself, inside and out. Altering the windows almost always requires listed building consent, and doing the work without it is a criminal offence, not a planning technicality. See our listed building cost page for what that does to a budget.
Speak to the conservation officer before you order anything. That conversation costs nothing next to a set of windows you have to take back out.
On a listed building, the conservation officer is your real customer. Get them on side before you order a single window, because if the sightlines are wrong they can make you take the whole lot out at your own cost.
Tom Bradley, window installer
Is it cheaper to restore a sash window than replace it?
Worth itIn most cases yes, and by a clear margin. A sash is a repairable object in a way a sealed uPVC unit is not: rot can be cut out and spliced, a sill replaced, a sash rebuilt, and none of it disturbs the box frame. Checkatrade's restoration cost guide, updated July 2026, breaks the common jobs down as follows.
| Job | Range | Typical |
|---|---|---|
| Sill splice repair | £120–£240 | £180 |
| Stile or front cheek repair | £120–£240 | £180 |
| Full sill replacement | £160–£320 | £240 |
| Draught-proofing | £248–£388 | £318 |
| Replacement sash | £270–£400 | £335 |
| Full refurbishment | £620–£920 | £770 |
Source: Checkatrade sash window restoration cost guide, updated July 2026.
Set the £770 typical refurbishment against a new timber sash from £1,450 installed, on the same source, and the case makes itself. A full refurbishment covers repairing timber, replacing cords, repairing the glazing, sanding, draught-proofing and repainting, at roughly one working day per window. Replacement wins when the frame itself has structurally failed, when rot is extensive rather than localised, or when the existing windows are twentieth-century replacements with no heritage value to protect. Our comparison of repair versus replacement sets out the framework, and the repair cost calculator will put a figure on it.
Why will my sash window not stay open?
On a traditional box sash, a snapped cord, nearly every time. The cord runs over the pulley to a weight in the box, and when it fails the counterbalance is gone, so the sash slams shut or hangs crooked. It is a cheap fix relative to the panic it causes: Checkatrade puts a single inner cord at around £70, an outer cord at around £80 because it is harder to reach, and all four cords in one window at around £150. If one cord has gone the other three are the same age, so have all four done in the same visit and pay for one set-up rather than four. On a modern sash with spiral balances the symptom is the sash creeping down slowly rather than dropping, and the fix is a replacement balance.
How do you stop a sash window rattling and draughting?
Rattle and draught are the same fault: the sash is loose in its channels. The remedy is draught-proofing, which fits discreet brush pile seals into the sash channels and along the meeting rail where the two sashes cross. Done properly the window still slides normally, the rattle stops, and the air leakage that made the room cold at ankle height goes with it. Checkatrade puts it at £248 to £388 per window, with £318 typical. For an original single-glazed sash this is the highest-value thing you can do for the money, and worth doing before you consider glazing at all, because no glazing fixes a gap.
Are uPVC sash windows any good?
It depends Judged as windows, modern uPVC vertical sliders are good: they run on spiral balances, tilt in for cleaning on most systems, seal far better than any original, need no repainting, and cost £600 to £1,500 fitted against timber at £1,330 to £2,850. On a 1930s semi with tired 1980s sashes, or a rental where the EPC matters more than the elevation, they are sound.
Judged as period windows they have real tells. Frame sections are thicker, so the glass area shrinks and the proportions thicken. Glazing bars are usually applied to the face of the glass or suspended inside the unit rather than being real dividers, so they read flat. Horns are moulded. And white uPVC has a sheen painted timber does not, which is why foiled heritage colours look markedly better on an old frontage.
The practical test is the pavement test: stand where a passer-by would and look at the whole elevation, not a showroom sample. If your terrace still has original timber sashes either side of you the difference will show, and in a conservation area it may not be permitted at all. Our uPVC versus timber comparison weighs it up in full.
How long do sash windows last and what upkeep do they need?
A maintained timber sash lasts 30 to 60 years, and plenty of original Victorian sashes are well past 120, because the design is repairable in pieces. That longevity is entirely conditional on the paint film. Paint is not decoration on a timber window, it is the waterproofing, and once it cracks at a joint water gets into end grain and rot starts where you cannot see it.
Checkatrade puts repainting a sash window and frame at £200 to £500 per window, and sets the interval at five to ten years depending on exposure, so a west elevation taking wind-driven rain needs attention sooner than a sheltered rear. Beyond painting the routine is undemanding: keep the pulleys and channels clear, ease a sticking sash rather than forcing it, and replace a frayed cord before it snaps. Our window aftercare guide covers the seasonal checks and the lifespan guide compares materials over the long run.
Do replacement sash windows meet Part L?
Yes, provided you buy a modern unit from a competent supplier. A replacement window in England must achieve a U-value of 1.4 W/m²K or lower, or a Window Energy Rating of band B or better, under Approved Document L. The work must be notified, either through a FENSA-registered installer who issues a certificate on completion, or to your local authority building control. You will want that certificate when you sell.
Two caveats specific to sashes. Retrofitting slim units into existing frames is a repair rather than a replacement window, so it is not assessed against the Part L standard in the same way and you should not expect it to reach 1.4 W/m²K. And on a listed building, conservation requirements can override the energy standard entirely, which is exactly what secondary glazing exists to solve.
On the saving, keep expectations grounded. The Energy Saving Trust puts the annual bill saving from installing A-rated double glazing throughout an entirely single-glazed semi-detached home at £85 a year in Great Britain and £140 a year in Northern Ireland. Against timber sash prices that is not a payback argument, and nobody should sell it to you as one.
The verdict on sash windows
It depends Sash windows are worth the premium when the house genuinely calls for them, on a Victorian terrace, a Georgian townhouse or anywhere the street and the planners expect vertical sliders. For a modern home, the casement is the better-value pick. If you are torn between the two styles, compare prices and looks against our casement windows guide, then price the job in the sash window calculator.
If you already have original sashes, the order of operations saves money. Draught-proof first, repair what is broken second, and only then choose between reglazing the sashes you have and replacing them.
Frequently asked questions
A fitted uPVC sash window costs £600 to £1,500 in 2026. Timber sash windows for period and conservation-area homes cost more, typically £1,330 to £2,850, reflecting the heritage frame and finish.
A sash window has one or two panels, called sashes, that slide vertically within the frame rather than swinging open. It is the traditional window of Victorian and Georgian homes across the UK.
Yes. Modern uPVC and timber sash windows are made with slim double-glazed sealed units that keep the period proportions while meeting the Part L requirement of a U-value of 1.4 W/m²K or lower.
Often yes. A specialist routs out the original rebate and fits slim-profile sealed units into the existing sashes, which Checkatrade prices at £650 to £1,300 per window. Historic England accepts this may be possible where a historic window survives without historic glass, but warns thicker units sometimes force renewal of the frame itself. On a listed building you need consent first.
Often not. Conservation areas and listed buildings usually require timber sashes that match the originals, and may need consent for any change. Always check with your local planning authority before ordering.
Sash windows carry more mechanism (the sliding sashes, balances or cords and the box frame) than a simple hinged casement, so they cost more to make and fit. A uPVC sash runs £600 to £1,500 against £400 to £1,000 for a uPVC casement.
They need repainting every 8 to 10 years to protect the frame, plus occasional easing of the sashes. In return they can last 30 to 60 years and are often the only style permitted on period and listed homes.
Almost always a broken sash cord. The cord runs over a pulley to a cast iron weight hidden in the box frame, and when it snaps the counterbalance is gone. Checkatrade puts a single inner cord at around £70 and an outer cord at around £80, with all four cords in one window at around £150. On a modern sash the equivalent failure is a worn spiral balance.
A traditional box sash counterbalances each sash with a cast iron weight on a cord running over a pulley inside a hollow box frame. A spiral balance replaces that with a spring-loaded tube in the side of the frame, which is why modern uPVC and slim timber sashes need no bulky box. Cords are repairable almost indefinitely; balances are sealed units that get swapped.
Checkatrade puts sash window draught-proofing at £248 to £388 per window, with £318 typical, as of July 2026. Discreet brush seals go into the sash channels and around the meeting rail so the window still slides normally. It is usually the cheapest meaningful comfort improvement you can make to an original sash.
Usually yes. Checkatrade puts a full sash window refurbishment at £620 to £920 per window, with £770 typical, against a new timber sash from £1,450 installed. Restoration also keeps the original timber and glazing bars, which is often the deciding factor on a listed building or in a conservation area.
Yes. A replacement window in England must achieve a U-value of 1.4 W/m²K or lower, or a Window Energy Rating of band B or better. Modern uPVC and timber sashes with slim double-glazed units meet this, and the work should be notified through a FENSA-registered installer or your local authority building control.