From a slim-frame glazed box off the kitchen to a fully frameless structural glass build. The engineering behind them is what decides whether they look extraordinary in year ten or leak, mist and overheat.
“Glass extension” covers three quite different builds at three quite different prices, and getting the label right is the first thing worth doing, because it moves the budget by a factor of five.
A framed glass extension uses slim aluminium or steel profiles — sightlines down to around 20 mm on good systems — carrying large panes between them. It is the most common London build, works on almost any house, and is the most forgiving thermally. A structural glass extension removes the frame almost entirely: the glass itself is the structure, panes bonded with structural silicone joints and often a glass beam overhead. It is the one people picture, it is spectacular, and it costs roughly 40–60% more per square metre. A glazed roof — a lantern or large rooflight over an otherwise conventional brick extension — gets a lot of the daylight benefit for a fraction of the money.
We build all three, and we will tell you at the site visit which one your house, your orientation and your budget actually point to. Quite often it is not the one people arrive asking for.
| Framed | Structural / frameless | Glazed roof only | |
|---|---|---|---|
| Typical cost | £2,000–£3,200 per m² | £3,200–£5,500 per m² | £1,600–£5,500 per unit |
| Frame | Slim aluminium or steel, ~20 mm sightlines | Structural silicone joints, almost no visible frame | Aluminium lantern or rooflight kerb |
| Thermal control | Most forgiving | Hardest — needs solar control glass and ventilation designed in | Easy — small glazed area |
| Site access | Panels usually carried in | Often needs a crane | Carried in |
| Best for | Most London houses, most budgets | High-value homes where the view is the point | Getting daylight into a brick extension cheaply |
Glass earns its premium in specific situations rather than universally. It tends to be right when:
South and west-facing glass boxes overheat badly without solar control glass, ventilation and often external shading. That is solvable, but it has to be designed in rather than discovered in July.
Two glass extensions can look identical on a drawing and perform completely differently. These are the decisions that matter, and the ones worth interrogating in any quote you are comparing.
A glazed extension is treated by planners like any other extension: the same permitted development limits on depth, height and boundary distance apply, and the same 45-degree rule protecting the neighbour’s daylight. Glass gets no special dispensation for being lightweight.
In conservation areas the picture is more nuanced, and often more favourable than people expect. Officers frequently prefer a frankly modern, visually lightweight glazed addition over a brick pastiche that pretends to be original. What they object to is a bad match, not a clear contrast. On listed buildings the same logic applies but listed building consent is required in addition to planning permission, and the junction between old fabric and new glazing gets scrutinised closely.
Building Regulations are the real constraint on a glass build, not planning. Part L limits how much glazing you can have before you must prove performance by calculation; Part O covers overheating; Part K covers safety glazing at low level and in doors. None of these stop you building a glass extension — they decide what specification it has to be, which is exactly why the glass make-up should be settled before the quote, not after.
On anything unusual — a conservation area, a listed building, or a design that pushes the envelope — it is worth paying for the council’s pre-application advice service before a full submission. It typically costs a few hundred pounds and tells you what the officer will object to while changing the drawing is still cheap.
The Party Wall Act and Thames Water build-over rules apply in the usual way where the build sits near a boundary or a public sewer.
Glazing type is the dominant variable — far more than floor area. These are 2026 London ranges:
| Scope | Typical London cost (2026) | Build time |
|---|---|---|
| Glazed roof only — lantern or large rooflight on a conventional extension | £1,600 – £5,500 supplied & fitted | Within main build |
| Framed glass extension — slim aluminium or steel-look, around 20 m² | £50,000 – £100,000 | 10–16 weeks |
| Structural / frameless glass box — same footprint, minimal frame | £80,000 – £180,000+ | 14–20 weeks |
Guide ranges for budgeting, not a fixed price list. Every project is quoted in writing after a free site visit.
Roughly, framed glazing runs £2,000–£3,200 per m² and genuine structural glass £3,200–£5,500 per m² in London. For the full type-by-type breakdown — including roof lanterns, glass side returns and where the crane and cleaning costs come in — see our glass extension costs by type guide → or the overall glass extension cost guide →
We visit your property, measure up, listen to your vision, and ask the right questions about what you need.
Written quote after the free site visit.
All work managed end to end by us. Daily progress updates if you want them, clean working practices throughout.
Final walkthrough, snagging, full clean. We leave your property spotless and stand behind every detail.
Not if it is specified properly, and unbearable if it isn’t. Modern double or triple units with a low-E coating hold heat in far better than people assume — the old conservatory reputation comes from single glazing and polycarbonate roofs. Summer is the harder problem: a south or west-facing glass box needs solar control glass, genuine cross-ventilation and often external shading. We size that against your actual orientation rather than fitting a standard spec.
More than brick, less than people fear. Self-cleaning coatings handle ordinary rain-borne dirt on sloped and vertical surfaces. Roof glass and anything above ground-floor reach needs professional cleaning once or twice a year, and access is what drives that cost — a cherry-picker on a tight London site is not cheap. Seals and gaskets should be inspected every few years.
Interstitial condensation inside the sealed unit means the unit has failed and is a warranty matter. Surface condensation on the inside of the glass on a cold morning is normal physics, worse in kitchens, and managed with adequate ventilation and extraction — which is why the mechanical ventilation design matters as much as the glass on a glazed kitchen extension.
Yes, and generally more so than a timber door. Laminated glass is extremely difficult to get through — it holds together under repeated impact rather than shattering. The weak point on any glazed elevation is the locking hardware on opening panes, so that is where the specification should go: multi-point locks and, where it matters, tested security-rated units.
The structure will outlast the sealed units. Well-made double-glazed units typically give 20 to 30 years before the seal degrades, and individual panes can be replaced without touching the structure. Aluminium frames and structural silicone joints, kept clean and inspected, last considerably longer.
Usually, but access has to be solved before the design is fixed, not after. Large panes may need craning over the roof, which means a road closure, a crane day and a trained crew — a real cost line, typically a few thousand pounds. Sometimes the sensible answer is a slightly different pane layout that can be carried through the house. We work that out at survey stage so it is in the quote rather than a surprise.
The same rules apply as for any extension — glass gets no exemption. The difference is that in conservation areas a modern glazed addition is often received more favourably than a brick imitation, because it reads as clearly new rather than as a poor copy. Listed buildings need listed building consent as well as planning permission.
No. Framed glazing runs roughly £2,000–£3,200 per m² against about £2,000–£3,000 for comparable brick, and structural glass is well above both. Where glass genuinely wins is daylight into a deep plan and the view out — not cost. If the budget is the binding constraint, a brick extension with a good roof lantern gets you most of the light for much less.
Sealed units carry a manufacturer’s warranty, typically 10 years against seal failure, and the frame system carries its own. Our workmanship is covered separately. We hand over the paperwork for all of it at completion, along with the cleaning and maintenance schedule — because several of those warranties depend on the glass actually being maintained.
We build glazed extensions right across the capital, with local extension pages for the boroughs we work in most.
Fill the side alley of a Victorian terrace to widen a dark galley kitchen into a full-width room.
Learn more →Side return and rear extension in one L-shaped build — one scaffold, one planning application.
Learn more →Rear, side-return, wrap-around and double-storey builds — the full picture in one place.
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