How much of an extension budget should I set aside for glazing?

For an extension with a substantial sliding or bifold door plus a rooflight, £40,000–£100,000 is a realistic guide for the glazing package, though this varies enormously with scale, glass specification and structural detailing such as corner glazing.

Should I involve my glazing supplier before or after the extension design is finalised?

Ideally before, or at least alongside your architect. Frame sizes and structural opening requirements for large sliding doors, rooflights or corner glazing often need to be confirmed before foundations are dug, so early involvement avoids costly changes once building work is underway.

Is post-free corner glazing worth the extra structural cost?

It depends on priorities. Post-free corner glazing gives a striking uninterrupted view but requires significant structural engineering to support the roof without a corner post. A slimmer visible corner post using standard sliding door framing achieves much of the openness for considerably less cost.

Why should windows, doors and rooflights be specified together on an extension?

Frame colour, sightline width and glass specification all read together visually and affect thermal performance as a whole. Specifying each element separately risks mismatched finishes or inconsistent glass tint between the door, windows and rooflight on the same elevation.

Cost & Budget Guides

Extension Glazing Budgets

Extensions with substantial glazing, such as a sliding door and a rooflight, typically run £40,000–£100,000 for the glazing package within the wider build.

3 min read

Extensions are where glazing decisions tend to be most ambitious, because a single-storey rear extension is often designed around opening the space to the garden with a large sliding or bifold door, a rooflight, and sometimes a run of corner glazing where two walls of glass meet without a visible post. As a guide, extensions with substantial glazing typically run £40,000–£100,000 for the glazing package, though this varies enormously with scale and ambition.

This guide is about how to think of glazing as one coordinated specification within an extension budget rather than several separate purchases, since the door, the rooflight and the windows all need to work together visually and thermally.

What a typical glazing package includes

Element	Example spec	Rough guide contribution
Sliding or bifold door	4-5m aluminium slider	£11,000–£20,000
Rooflight	Large format fixed or opening	£4,000–£12,000+
Windows	2-4 aluminium windows to match	£3,000–£10,000
Corner glazing (if included)	Bespoke post-free corner junction	Significant additional cost

Example extension glazing package (illustrative only)

These figures are illustrative to show how the elements add up, not a quotation for any specific extension. A survey and design discussion is what turns this outline into a real specification.

Designing glazing as one specification

The most common mistake in extension glazing budgets is pricing the door, the rooflight and the windows in isolation, then discovering they do not sit well together aesthetically or thermally. Frame colour, sightline width and glass tint should be considered together across the whole extension, and it is worth having one point of contact for the full glazing package where possible.

Match frame colour and finish across door, windows and any glazed screens
Consider glass specification together: a solar control door and a clear rooflight can look and perform inconsistently
Think about sightline width holistically, especially where a sliding door meets a window on the same wall
Coordinate with your builder early on structural openings and upstand construction for the rooflight

A note on corner glazing

A post-free glazed corner, where glass meets glass without a visible structural upright, is one of the most striking details available on a contemporary extension, but it is also one of the most expensive because of the structural engineering required to support the roof load without a corner post. If budget is tight, a slimmer visible corner post at standard sliding door pricing achieves much of the same openness for considerably less outlay.

A structural corner window removes the post that would normally support the roof at the junction.

Sequencing glazing with the build programme

Glazing is usually one of the last elements installed on an extension, once the roof is on and the building is weathertight, but frame sizes and structural opening requirements need to be finalised early, often before the foundations are even dug. Involving your glazing supplier at design stage, alongside your architect or builder, avoids costly late changes to structural openings once brickwork is underway.