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 double-glazed sealed units: standard ones meet today's energy rules, and slimline double glazing keeps the period look at some cost to the U-value. 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.
Types of sash windows: the six styles you will actually be quoted
Sash is the name of the sliding panel, not of one window, so several different things get sold under it. These are the six you will meet on a quote, and the differences change both the price and what a planner will accept.
- Box sash. The traditional article. Two sashes on cords and lead or cast iron weights, hidden in the hollow sides of the frame. In Scotland it is called a sash and case window; it is the same window. That box is why original frames are so deep, and why the system can be repaired part by part.
- Spring or spiral balance sash. The modern build, patented in 1930 according to Historic England. A spring in a tube, fitted in the side of the sash or the frame depending on the system, replaces the weights, so the frame can be much shallower. uPVC sashes use it as standard, and timber makers offer it alongside traditional weights.
- Vertical slider. What most uPVC makers call their sash window. Engineered from the start as a slider rather than copied from a box sash, usually with tilt-in sashes for cleaning and mock horns for the period look.
- Double-hung. Both sashes move. This is what people normally picture, and it is the version that lets you open the top and bottom together to ventilate a tall room. Historic England dates the earliest surviving double-hung sash to 1701.
- Single-hung. Only one sash moves. It is the oldest form: Historic England says the earliest vertically sliding sashes had a fixed top sash, with the lower one wedged or pegged open.
- Yorkshire horizontal slider. The sliding idea turned on its side, and a window wider than it is tall. Historic England says these were used widely, not just in Yorkshire. Historic Environment Scotland says that strictly it is not sash and case at all, because the sash is pushed by hand rather than counterbalanced, so there are no weights or cords.
Why is it called a sash window?
Sash comes from the French châssis, meaning a frame. It arrived in English in the seventeenth century describing the glazed frame that carries the panes, and it stuck to the window as a whole once the sliding version became the standard English pattern.
What do six-over-six and two-over-two mean?
The numbers count the panes in the upper sash and then the lower one, and the pattern dates the window as reliably as anything else on the elevation. Six-over-six is the Georgian arrangement, with slim glazing bars dividing small panes because large sheets of glass were not available. Two-over-two is Victorian, once plate glass allowed bigger panes and the bars retreated to a single vertical division. One-over-oneis late Victorian and Edwardian, a clear sheet in each sash. Either side of 1900 a small-paned upper sash over a one or two pane lower sash was popular too, part of what Historic England calls the Queen Anne revival's renewed interest in small panes and thick glazing bars.
Two details decide whether a replacement looks right. A true glazing bar (an astragal, in Scotland) divides real separate panes, while an applied bar is stuck to the face of one large sealed unit to imitate them, and the giveaway is the shadow line. Historic England says stuck-on bars on a historic window are unlikely to be acceptable. A run-through horn is a horn moulded as a continuation of the stile rather than glued on, which is the detail heritage officers look for on a like-for-like replacement.
How do you date a sash window?
Three things date a sash without any paperwork. Horns, the small curved projections at the base of the upper sash, came with heavier plate glass, which needed the corner joint reinforced. Historic England ties them to the cheaper plate glass of the 1830s onwards in its text, and to the late 19th century in a caption; in Scotland, Historic Environment Scotland says horns are not very common and often do not appear until late in the century, if at all. So a sash with no horns and small panes is usually earlier. The glasstells you next: crown and cylinder glass ripple and distort the view (crown with curved ripples, cylinder with straight ones, on Historic Environment Scotland's test), while modern float glass is optically flat. Taxes shaped the panes: the window tax, first imposed in England in 1696, was repealed in 1851, and the excise duty on glass was abolished in 1845, when Sir Robert Peel's Budget proposed its repeal. And the glazing bars: Historic England says early bars were often almost 40 mm thick, while by the early 19th century some were only 12 mm wide. Victorian sashes mostly lost their bars rather than thinning them, and the Queen Anne revival at the end of the century brought back small panes and thick bars, so a chunky bar is not proof of age. If you are pricing like-for-like replacements, take a close-up photograph of the horn and the bar profile before anyone quotes.
How does a sash window work?
A box sash has two sashes that slide past each other in the frame. Each hangs on cords that run over pulleys near the top of the sides of the frame to lead or cast iron weights in the hollow box behind, which counterbalance it; a spiral balance does the same job with a spring and no weights. A sliding sash window is the same thing described by its action rather than its frame. The labelled diagram, the balancing rule and the parts that wear out are in our guide to the parts of a sash window.
You can open the top and bottom together to ventilate a tall room. The trade-off is more mechanism than a casement, which is why a sash costs more to make and fit than the simpler hinged style covered in our casement windows guide.
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.
Double Glazing Cost Calculator Team
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. For whole-house totals and London prices, see what double glazed sash windows cost.
| 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.
Double glazed sash windows: what you actually get
A new double glazed sash is not the same product as the slim unit retrofitted into an old frame, covered further down, and the two get confused constantly. On a new window the question is which sealed unit the price includes, because that decides both the U-value and how the window looks from the street.
A standard 28mm unit is the normal double glazed sandwich: two panes with a cavity between them, and the specification that comfortably meets the Part L requirement of 1.4 W/m²K or better for a replacement window. A slim-profile unitsqueezes the same idea into a much narrower cavity so the sash keeps a period sightline. It looks better on an old house and it performs worse: makers declare 1.1 to 2.1 W/m²K at the centre of the glass, and when Historic Scotland measured installed units most came in near 2.0. Ask for the specific U-value in writing rather than the phrase energy efficient.
Two things to check before you sign. Ask whether the unit has a warm edge spacer rather than a plain aluminium one, since the spacer is where heat escapes at the perimeter and where misting starts when a seal fails. And ask what the sightline becomes, because a deeper unit needs a deeper rebate and the meeting rail thickens: on a Victorian frontage that is the difference between a convincing window and an obvious one. Price your own openings in the sash window calculator.
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. Our retrofit double glazing cost guide sets that against new sashes in the old box and secondary glazing.
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, and its Advice Note 18, revised in February 2026, goes further: slim-profile or vacuum double glazing within historic frames "will generally be acceptable". 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, and frames or glazing bars that cannot take the units are one of its exceptions. 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: Historic Scotland found makers' figures generally overestimated what the units achieved in real sashes. The weak point is the edge seal, where the Glass and Glazing Federation has linked early failures to spacers and sealant cut back to hide the unit's edge behind a slim bar. Our slimline double glazing guide sets out the measured figures. 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 listed, or sits in a conservation area where the council has made an Article 4 direction, you usually cannot simply fit whatever you like. Planners often require timber sashes that match the original proportions, glazing bars and sightlines, and you need consent or planning permission before any work starts. An unlisted house in a conservation area with no Article 4 direction is freer: the Planning Portal says new windows of a similar appearance to those the house was built with do not usually need an application. 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.
Double Glazing Cost Calculator Team
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. On the interval its own guides vary: five to ten years depending on exposure in its restoration guide, five to eight in its timber windows guide, and ExpertSure says 3 to 5 years for softwood and 5 to 7 for hardwood. Plan on every 5 to 8 years, sooner on a west elevation taking wind-driven rain than on 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 not a replacement window: Approved Document L paragraph 10.6 says replacing glazing in an existing frame is not a controlled fitting and "does not need to meet the energy efficiency requirements", so 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 downsides of sash windows
The case against is rarely put honestly by anyone selling them, so here it is. None of these are reasons to avoid a sash on a house that wants one, but they are the things people wish they had known.
- They cost more than a casement, permanently. More mechanism, more joinery and more labour to fit. The premium does not go away on the second set.
- Timber sashes need painting on a cycle. Repainting runs £200 to £500 per window every five to eight years, and the paint film is the waterproofing rather than the decoration. Skip it and rot starts in end grain where you cannot see it.
- Originals are draughty by design. A sash has to slide, so it cannot seal like a compression-fitted casement. Draught-proofing closes most of the gap but not all of it.
- The mechanism is a wear part. Cords fray and snap, spiral balances weaken and let the sash creep down. Both are cheap fixes and both are jobs a casement never needs.
- Slim units carry more risk. Keeping the sightline low means less sealant at the edge of the unit, which the Glass and Glazing Federation has linked to early failures, so read what the guarantee covers if it mists rather than how long it runs.
- Fewer installers can do the work properly. Heritage sash work is a specialism, and on a period or listed house the cheapest quote is often from whoever understands it least.
- They are wrong on the wrong house. On a modern semi a sash reads as pastiche and costs more than the casement it is imitating.
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.
Get quotes for sash windows
The ranges above are the benchmark: £600 to £1,500 fitted for a uPVC sash, more for timber. For a price on your own openings, tell us what you need and Leads 2 Trade will arrange quotes from installers covering your postcode.
Two things worth doing when the quotes land. Ask whether the price is for a slim-profile double glazed unit or a standard 28mm one, because on a sash that choice changes the sightline as well as the price. And if the house is period or listed, ask what the installer has fitted nearby before you judge the number.
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 with standard double-glazed sealed units can meet the Part L requirement of a U-value of 1.4 W/m²K or lower. Period-style sashes glazed with slimline units keep the original proportions but generally cannot reach 1.4, so a new window glazed that way usually relies on the heritage exceptions in Approved Document L.
Often yes. A specialist fits sealed units into the existing sashes: slim-profile units, standard argon units where the rebate is routed deeper, or vacuum units. Checkatrade prices fitting double glazing into existing sash frames at £650 to £1,300 per window, though it does not say the units are slim. Historic England's Advice Note 18, revised in February 2026, says slim-profile or vacuum glazing within historic frames will generally be acceptable, except where historic glass of interest survives or the frame cannot take the units. On a listed building you need consent first.
It depends on the street. On a listed building you need consent, and Historic England and council guidance are against uPVC. An unlisted house in a conservation area usually needs no planning application for new windows of a similar appearance to the originals, unless the council has made an Article 4 direction removing that right. Where an Article 4 direction applies, or the property is a flat, you need planning permission, and councils differ: some generally refuse uPVC, others accept a carefully detailed uPVC sash. 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 5 to 8 years to protect the frame, sooner on an exposed elevation, 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 lead or 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 lead or cast iron weight on a cord running over a pulley in the side of the frame, the weight hanging in the hollow box behind. A spiral balance replaces that with a spring in a tube, fitted in the side of the sash or the frame depending on the system, which is why modern uPVC and slim timber sashes need no bulky box. A snapped cord is a cheap repair; a worn balance has to be replaced with one that matches the original's tube diameter, length and weight-bearing capacity.
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 standard double-glazed units meet this, and the work should be notified through a FENSA-registered installer or your local authority building control. Where slim units are needed to keep a building's character, Approved Document L accepts a centre-pane U-value of 1.2 instead, and reglazing an existing sash is not covered by the energy requirements at all.