Sashes and casements are the two window archetypes of British housing, and the choice between them is rarely about which is objectively better. It is about what suits the age of the house, the way the room is used, the budget available and the rules that apply to the elevation. This guide sets out what actually separates the two.
A short history of each style in the UK
The vertical sliding sash arrived in Britain in the late 1600s and became the default on the Georgian, Victorian and Edwardian terraces that shape the character of most UK town centres. The classic box sash sits proud of the brickwork, its cords and weights hidden inside the box, its glazing bars dividing large panes into recognisable proportions. Sashes remain the correct answer, culturally and often legally, on pre-1920 elevations.
The side-hung casement is older still in vernacular buildings, but its dominance in the modern British house dates to the 1930s. As inter-war semis rolled out across suburbia, the casement (a hinged sash swung from a frame) became the standard because it was cheaper to make, easier to open outward without curtain interference, and simpler to draught-seal. Casements suit 1930s-onwards housing, rural cottages, most post-war stock, and virtually every new-build since 1980.
How each opens and how you clean it
A casement swings outward on side hinges (occasionally top-hinged as an awning), which means the entire pane is out of the way when open. Cleaning the outside from inside is straightforward on a ground floor and possible on an upper floor if you can reach through, though not comfortably. Tilt-and-turn variants tilt inward from the bottom for ventilation and turn on side hinges for full opening and cleaning access.
A vertical sliding sash moves either the upper, lower or both sashes past each other on a track. Traditional counterweighted box sashes are elegant but require servicing every 15 to 25 years to renew the cords and pulleys. Modern spiral-balance sashes replace the weights with a spring mechanism and need less upkeep. Most modern sashes offer a tilt facility, letting each sash rotate inward for cleaning from inside; this is close to essential on anything above a first floor.
Thermal performance side by side
On paper, casements have a small advantage on thermal performance. A modern A+ rated UPVC casement sits comfortably at a whole-window Uw of 1.2 W/m²K, sometimes 1.0 with triple glazing. A UPVC vertical slider tends to sit at Uw 1.3 to 1.4 W/m²K because the mid-rail meeting point is inherently harder to seal against draughts than a compression-sealed casement frame.
The gap matters less in practice than the numbers suggest. A well-manufactured modern sash with brush seals, warm-edge spacers and argon fill will still hit Building Regulations Part L 2021 (Uw 1.4 W/m²K on replacements) without difficulty. Timber sashes, when properly draught-proofed, are only slightly behind UPVC in real-world energy performance. If you already own timber sashes on a period property, retrofit draught-proofing plus secondary glazing often outperforms a full replacement on both cost and character grounds.
Security ratings
The relevant benchmark in the UK is PAS 24, the enhanced security standard also required by Approved Document Q on new builds. Modern casements achieve PAS 24 easily with multipoint locking, laminated glass and mushroom-cam espagnolette hardware. Sashes are trickier: the sliding mechanism requires purpose-designed sash locks, keeps and reinforced meeting rails to meet the standard. Reputable sash manufacturers offer PAS 24-certified units, but always confirm this in the quote. Ground-floor and accessible sashes should be specified as PAS 24 as a matter of course.
Cost comparison
The single biggest factor in the sash-versus-casement decision is price, and the gap is larger than most homeowners expect until they compare quotes. Below are typical 2026 UK fitted prices, including FENSA or CERTASS notification and A+ glazing.
| Factor | Casement | Vertical slider sash |
|---|---|---|
| UPVC fitted price per window | £450 to £900 | £900 to £2,400 |
| Timber fitted price per window | £900 to £1,800 | £1,800 to £3,200 |
| Typical Uw (2026 standard) | 1.2 W/m²K | 1.3 to 1.4 W/m²K |
| Draught sealing | Compression seal, excellent | Brush seal, good |
| Cleaning outside from inside | Ground floor easy, upper hard | Tilt-in mechanism makes it easy |
| Security to PAS 24 | Straightforward | Achievable, but confirm |
| Typical property fit | 1930s onwards, cottages, new-builds | Georgian, Victorian, Edwardian |
| Conservation acceptability | Rarely on period front elevations | Standard for period elevations |
The rule of thumb is that a sash costs roughly twice a casement in the same material and glazing spec. On a full-house replacement that gap can be £3,000 to £8,000 depending on window count.
Planning and conservation implications
Casements and sashes are not interchangeable in the eyes of a conservation officer. If your property sits in a conservation area, on an Article 4 Directed street, or is listed at Grade II or above, swapping traditional timber sashes for UPVC casements is very likely to require planning permission and is very likely to be refused on the front elevation. Even where planning is not required (most side and rear elevations of non-listed period homes), a swap changes the appearance of the building and can reduce resale value on similar streets.
Conversely, retrofitting sashes onto a 1970s semi that has always had casements is legally straightforward but visually incongruous. The cost is real and the aesthetic gain is debatable.
When to keep, when to swap
Not every window needs replacing to perform. Consider retaining and improving in the following cases.
- Original timber sashes on a Victorian or Georgian house, where draught-proofing plus secondary glazing can hit Uw 1.6 or better while preserving the character and value of the property
- 1930s steel casements (Crittall style) where refurbishment plus slimline sealed units is available and often preferred to plastic replacements
- Any window under 20 years old that is A rated and still sealing correctly
Replace when the frame is failing, the sealed units are misted, the timber is rotten beyond viable repair, or the room is uncomfortable to use in winter despite adequate heating.
The material choice within each style
Both styles are available in UPVC, timber and aluminium. Timber costs roughly double UPVC in casement form and 60 to 80 per cent more in sash form, but is required or strongly preferred on most period and conservation properties. Aluminium is largely a casement product in the UK market, popular in contemporary extensions and rear elevations, and rarely used in sash form. Timber-effect UPVC (flush sash) offers a period aesthetic at UPVC prices and is often the pragmatic compromise on rear elevations of period homes where full timber is unnecessary.
A short decision framework
Sash if your house predates 1920, your elevation faces a public street, or you sit in a conservation area. Casement if your house is 1930s or later, if the elevation is not visually sensitive, or if budget is the binding constraint. Where you have flexibility (rear elevations, side extensions, or non-listed period homes), a flush UPVC sash gives most of the sash aesthetic at close to casement cost.
Mixed schemes are common and sensible. Many UK homeowners on Victorian terraces keep or replace timber sashes on the front elevation, where the street view matters, and specify UPVC casements or flush sashes on the rear kitchen and bedroom windows, where cost and thermal performance are the priority. Local planning departments are typically comfortable with this split, provided the front elevation stays true to the original detailing (glazing bar profile, horn detail on the top rail, correct pane proportions).
Getting the detailing right matters more than the style
Two sashes made by different manufacturers can look strikingly different from the pavement. The details that carry the eye are the width and profile of the glazing bars, the projection of the sill nose, the depth of the meeting rail, and whether the top-sash horns are correctly shaped. On casements, the equivalents are the sightline width, the presence or absence of a dummy sash bar, and the reveal depth on the reveal against the brick.
Ask any installer to show you the sightline drawing for the specific unit they are quoting, not a stock image. Photographs of previous jobs in similar property types are worth more than showroom samples, because they show how the frame reads at street distance and in real light rather than under retail lighting.
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