French Windows Explained

A French window is a pair of casement sashes hinged at the outer edges and meeting at the centre, with no fixed post or mullion between them. When both sashes swing outward together, you get a clear, full-width opening with nothing obstructing the view or the airflow. The window sits in a standard wall aperture, above floor level: that is the key point that separates it from a French door, which runs from floor to frame-head and lets you step through into the garden. If you need access to a patio or balcony, you want a door, not a window. If you want light, ventilation, and a clear view from a wall that stays at sill height, a French window is the product to price. Use the French window calculator to get a 2026 range for your opening.

What makes a French window different from a standard casement

In a standard casement window, a vertical mullion divides the frame into two independent sashes, each operating on its own. In a French window, that central post is removed entirely. Both sashes hinge outward from the outer edges of the frame, and when closed, they meet at the centre and lock together. The result is a single, unified pane of glazing across the full width of the opening, with no obstruction in the middle.

This matters most when both sashes are open. A standard two-light casement window ventilates well but the central mullion stays fixed, so you always have a structural element dividing the view. A French window, by contrast, opens to the full frame width: you see the garden uninterrupted, and the airflow is correspondingly better. For a dining room or kitchen extension facing a south-facing rear garden, that difference in openness is exactly why homeowners choose French windows over standard casements. If you are undecided on which style fits your elevation, the uPVC versus aluminium guide covers the frame options and their visual impact in more detail.

The clients who choose French windows nearly always want one thing: they want to open up the rear wall of a room without fitting a door unit and cutting into the floor. Once I explain that the cill sits at a normal sill height and you cannot walk through it, a fair number switch to French doors instead. Worth being absolutely clear on that before you commission the opening.

Tom Bradley, FENSA-registered installer

French windows versus French doors: clearing up the confusion

The naming causes genuine confusion, and it leads to expensive mistakes. Both products use paired sashes that open from the centre with no central mullion. The difference is entirely in where the opening sits. A French window occupies a standard window aperture: there is a cill, there is structural brickwork or blockwork below it, and the bottom of the glazed unit sits above floor level. A French door fills a floor-to-ceiling opening: the threshold is at or near floor level, and you can step through it.

If the purpose is access to a garden, patio, or decking, the right product is a French door. A uPVC French door pair runs roughly £900 to £2,950 fitted, with around £1,400 typical. French windows, at £1,100 to £2,200 for a uPVC pair, cost a comparable amount but serve a different function: they bring light and ventilation to a room without requiring a structural alteration at floor level. If you currently have a standard window opening and you want to preserve the brickwork below the sill, a French window is the simpler, less disruptive solution.

French window costs in 2026

All prices below cover a standard pair, supply and fit, including removal of the existing window, sealing and making good, and the FENSA certificate. Opening size is the main variable within each range: a narrower, lower pair will come in towards the bottom; a wider or taller pair moves towards the top. Aluminium costs more than uPVC primarily because the thermal-break extrusion and the slim-profile engineering carry a higher manufacturing cost. Timber sits at the top of the range because it is a labour-intensive material that requires careful joinery and finishing.

Source: DGCC 2026 dataset, cross-checked against Checkatrade and MyJobQuote.

Which homes suit French windows

Best Buy French windows work well in modern homes, post-war semis and terraces, contemporary rear extensions, and any room where a garden-facing wall has a standard window opening and you want to maximise light and ventilation without creating a door unit. Ground-floor rear rooms are the most common application. The outward-opening sashes need clear, unobstructed space outside to swing: a shrub or a raised flowerbed right against the wall will cause problems.

Upper-floor installations are possible but less common. If the internal floor level is within 800 mm of the cill, you will need a Juliet balcony railing or another compliant safety barrier in front of the opening under Part K of the Building Regulations. For very wide spans where you want glass running along most of a rear wall, a bay window is a different structural approach that adds projection as well as width, at a higher cost and with more building work involved.

It depends On a period frontage or within a conservation area, check with your local planning authority before fitting French windows to the front elevation. The outward-opening mechanism and the absence of a central mullion can conflict with the character of a Victorian or Edwardian street, and conservation officers may not permit the change. Rear installations in those same areas are almost always unproblematic under permitted development.

In twenty years I have fitted French windows almost exclusively on rear elevations. Nobody wants outward-opening sashes swinging over the pavement, and conservation officers will not allow a mullion-free casement profile on a Victorian streetscape. Rear dining rooms and garden rooms, yes, every time. Front elevations, it is not worth the conversation with planning.

Tom Bradley, FENSA-registered installer

Choosing the right material for French windows

uPVC is the practical starting point for most homes. The frames are thermally efficient, require almost no maintenance beyond an occasional wipe and a drop of oil on the hinges, and the price range of £1,100 to £2,200 per pair is considerably lower than the alternatives. White remains the standard colour; coloured foils in anthracite grey, black, or other finishes add a modest premium and are increasingly common on contemporary extensions.

Aluminium makes sense when the frame sightlines matter as much as the glass area. The frames are slimmer, the profiles sharper, and the overall look more architectural. For a contemporary rear extension with large panes and a flush external finish, the extra outlay on aluminium can be justified by the visual result. The uPVC versus aluminium comparison sets out the U-value performance, expected lifespans, and typical cost differences side by side if you are weighing the two.

Timber is a specialist choice, reserved for Victorian, Edwardian, and Georgian properties where the frame profile and material are expected to match the original character of the building. Conservation officers will often specify timber as a condition of consent. The maintenance commitment is real: timber frames need repainting every eight to ten years to prevent moisture ingress and frame degradation. If you are not bound by conservation constraints, the premium of £2,090 to £4,180 per pair is difficult to justify over uPVC or aluminium on purely practical grounds.

The verdict on French windows

For a rear dining room, garden-facing extension, or any room where you want a wide, unobstructed opening at window height, a French window is the correct product. uPVC at £1,100 to £2,200 per pair is the sensible default for most UK homes: thermally competent, low-maintenance, and well within budget. Step up to aluminium at £1,490 to £2,970 when slim sightlines and a sharper contemporary profile matter. Reserve timber for period properties and conservation-area installations where the planning rules or the building character make it the appropriate material.

If what you actually need is access to the garden, price a French door pair via the door cost calculator: uPVC French doors run from £900 to £2,950 fitted, with around £1,400 typical, and they are built for floor-level passageway openings rather than wall apertures. For the standard single-sash alternative, the casement windows guide covers costs and suitability across the same three materials. The full guides hub lists every window style and cost guide on the site.