Glass Extension Costs by Type: Roof, Box, Frameless (London 2026)

“Glass extension” isn't one product with one price — it's at least four, and they don't cost remotely the same. A roof lantern dropped into a normal brick extension might set you back a couple of thousand pounds. A full frameless glass box is a different universe: six figures for a modest footprint. This guide prices each type separately in 2026 London terms, so you can find the one you're actually planning and see a realistic number.

If you've read our broader glass extension cost guide — which answers “what is it” and “is it worth it” — this post is the companion that breaks the cost down by sub-type. Below is the headline comparison, then a dedicated section for each type with what drives its price. All figures are 2026 London-weighted (roughly 20–30% above national averages once access, crane hire and London labour rates are factored in) and are quoted before VAT unless stated.

1. Glass roof / roof lantern only

This is the cheapest way to get “glass extension” light, because the extension itself is a normal single-storey brick build — you're just glazing the roof, not the walls. A roof lantern (the pitched, raised glazed box you see over kitchen islands) or a large fixed rooflight is supplied and fitted for roughly £1,600–£5,500 in 2026, depending on size and frame.

Breaking that down: a small aluminium lantern starts around £1,280–£1,650; a mid-size unit runs £1,880–£2,300; and a large or bespoke lantern comfortably exceeds £6,000. Most homeowners adding a lantern to an existing flat extension roof spend £2,500–£4,500 fitted, which includes forming the timber kerb it sits on plus flashing and trims.

The number climbs when structural work enters the picture. If a new structural opening has to be formed in the roof, add roughly £400–£1,200 for the timber or steel plus Building Control sign-off. And once scaffolding, internal plastering and finishing are added on a larger project, builders report the all-in figure for a substantial glazed roof can push past £10,000. The lesson: the lantern unit is rarely the expensive part — the opening, kerb and making-good around it are.

Choose this route if you want a bright, airy rear extension but the walls being solid brick doesn't bother you. You get most of the daylight benefit of a glass extension for a fraction of the cost.

2. Full glass box / structural glass extension

This is the type most people picture when they say “glass extension” — a box where the walls and roof are glass, held together with minimal or no visible frame. It is also the most expensive category by a wide margin. Expect £3,200–£5,500 per m² in London for 2026, versus around £2,000–£3,000 per m² for a comparable brick extension.

In whole-project terms, a typical 20m² framed glass extension lands at roughly £50,000–£100,000, while a structural/frameless box of the same size runs £80,000–£180,000+. The gulf comes from the engineering: with no steel frame doing the work, the glass panes themselves are structural — thicker, laminated, made to tighter tolerances, and craned into place. On a mid-terrace London property with no side access, that crane and hoisting alone adds several thousand pounds because every pane comes in over the roof.

A big chunk of the spread also comes from what's inside the box. If the glass extension houses a new kitchen, add £15,000–£45,000 depending on spec — that's often the single largest line after the glazing. Whether a glass box is “worth it” over a cheaper brick-plus-lantern build is exactly the question our main glass extension guide tackles.

3. Frameless vs slim-frame glazing — the spec that swings the bill

Within any glass extension, the glazing specification is the biggest single lever on price, and it's where two quotes for “the same” extension diverge by tens of thousands. The choice sits on a spectrum from budget-friendly framed units up to purist frameless.

Frameless / structural glazing uses structural silicone joints instead of frames, so panes appear to meet at glass-to-glass corners with no visible mullion. It's the most striking finish and typically costs 40–60% more than framed glazing, landing at roughly £3,200–£5,500 per m² of build. Slim-frame aluminium — the popular middle ground — keeps thin, crisp sightlines at a lower cost than true frameless. Standard framed units (chunkier bi-folds and sliders) are the most affordable and can bring a glass build closer to £2,000–£3,000 per m².

A practical takeaway: if the frameless look is driving your budget over the edge, slim-frame aluminium delivers most of the visual lightness for meaningfully less. It's the specification most London glass side-returns actually get built in, not full frameless.

4. Glass side-return extension (Victorian terrace)

The classic London job: fill the narrow L-shaped side return beside a Victorian or Edwardian kitchen with a glazed roof, a glazed gable, and sliding or bi-fold doors to the garden. It's the type most TrustBuilt enquiries are about. Typical 2026 London figures are £45,000–£95,000, with all-in projects (build, new kitchen-diner, glazing, professional fees and 20% VAT) reaching £70,000–£120,000 at roughly £2,800–£5,500 per m².

Spec level is what moves the number within that range. A basic glass side-return (standard bi-fold or sliding doors, mid-range kitchen, EPDM flat roof with rooflights) runs about £35,000–£50,000. A mid-spec build (slim-frame aluminium glazing, premium kitchen, a roof lantern, engineered oak flooring) is roughly £50,000–£65,000. A high-spec build (a structural glass corner, bespoke kitchen, stone worktops, designer lighting) is £65,000–£80,000+.

The terrace-specific cost to plan for is access. With no side alley, materials and glass come in over the roof — budget £3,000–£8,000 extra for crane hire and hoisting on a mid-terrace, plus parking-bay suspensions for skips and deliveries. That's a genuine London premium, not padding.

What drives the price within every glass type

Whichever type you're pricing, the same handful of variables decide where you land inside its range:

• • Framed vs frameless glazing — the single biggest lever, worth a 40–60% swing on the glass itself.
• • Whether a kitchen goes inside — adds £15,000–£45,000 and is often the largest single line.
• • Site access — mid-terrace with no side alley means crane/hoist hire (£3,000–£8,000) and slower, costlier labour.
• • Structural opening — how much rear or return wall comes out, and the steel needed to hold the house up.
• • Ground conditions — London clay in north/west boroughs often needs deeper or piled foundations.
• • Glass performance — large panes, solar-control coatings and triple glazing all add cost and sometimes crane time.

How to price YOUR specific glass extension

Start by identifying which of the four types above you're actually building — that alone narrows a “glass extension” from a £2,000-to-£180,000 question down to a workable range. Then place yourself within it on spec (framed vs frameless), kitchen (in or out), and access (side alley or crane). For a fast ballpark tailored to your footprint, our cost calculator gives an instant 2026 London estimate. Just remember that online calculators use averages — for glass, the frameless-vs-framed decision can move the real figure more than the square-metre count does, so treat any tool as a starting point and get a written quote before you commit.

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