Question 1

Are glass extensions always freezing cold in winter?

Accepted Answer

The myth compares modern structural glazing with a 1980s single-glazed conservatory. They are different products wearing the same word. 28 mm double glazing with a low-E coating sits at U ≈ 1.2–1.4 W/m²K — inside Building-Regulations territory, not conservatory territory. A 27 m² glass box at U 1.3 with a 23 °C indoor–outdoor design difference loses 1.3 × 27 × 23 ≈ 807 W — one oil-filled radiator on its middle setting holds it. The same room in single-glazed 6 mm glass (U ≈ 5.8) loses ≈ 3,600 W — 4.5× the heat, all day, every day. The glass did not change between those two rooms. The specification did — and the specification is priced in the calculator above.

Question 2

Do glass extensions always need planning permission?

Accepted Answer

Not always — but the honest answer carries three conditions, and the rules are set locally, so treat this as orientation rather than instruction. In England, many single-storey rear extensions fall under permitted development: up to 6 m deep for attached houses and 8 m for detached homes (the larger-home route needs prior approval from the council). Height limits apply — typically 4 m overall, or 3 m within 2 m of a boundary — and side-facing windows above 1.7 m must be obscure-glazed. Flats, maisonettes, conservation areas and Article 4 directions remove permitted rights entirely; listed buildings need consent for anything. One 20-minute check of your local planning portal settles it for your exact plot before you budget a penny for applications.

Question 3

Glass roofs always leak — true or false?

Accepted Answer

False — but the myth earns its keep, because every real leak traces to a detail that was bought cheap or never detailed. Modern overhead glazing seals twice: EPDM gaskets carry the water, and structural silicone — rated for 25+ years — carries the back-up. Leaks cluster at exactly three details: roof pitch under 3°, unsealed timber upstands, and the cap-to-wall junction where flashing was skipped. A roof laid at 5° or steeper, on a 150 mm insulated upstand, with a properly flashed wall junction, is a solved engineering problem — installed thousands of times a year. Put the pitch and the junction detailing into the contract documents, not into a verbal assurance, and the leak myth stays a myth.

Question 4

Does a glass box cost far more than a brick extension?

Accepted Answer

The premium is real but smaller than gossip suggests — and part of it is an accounting illusion. Like-for-like in London: a structural glass box runs £2,400–3,600/m² against £2,200–3,200/m² for a quality brick extension — roughly a 10–15% premium. Glass quotes exclude plaster, skirting and decoration (£150–300/m²) that brick-build quotes usually include; strip the comparison to finished rooms and the true gap narrows to ≈ 5%. The premium buys the roof structure and the laminated elevations themselves — walls that are glass replace walls that were the cheapest part of a brick build.

Question 5

Does the summer sun make glass rooms unusable?

Accepted Answer

Only if you buy the wrong coating — overheating is a specification choice, not a property of glass. Standard low-E glass passes most of the sun's heat: solar heat gain coefficient G ≈ 0.63. A solar-control coating drops G to ≈ 0.27 — a 57% cut in heat gain — for roughly 8–12% more on the glass package. Add a 500–600 mm roof overhang: the high UK summer sun (55–60° at noon) is blocked while the low winter sun (15–20°) still enters and warms the room for free. Roof vents or an opening rooflight purge hot air and are worth 2–4 °C on sunny afternoons — price them in, they cost hundreds, not thousands.

Question 6

Does every glass box need steel structure everywhere?

Accepted Answer

No — glass is a far better structure than the myth allows. 15–19.5 mm laminated glass fins and wall panels carry typical 2.5–2.8 m tall boxes over spans to ≈ 3.0–3.6 m with no posts at all. Steel appears where spans stretch past ≈ 3.6 m, at point-supported corners and at parapets — typically £2,500–8,000 of a London budget, not the whole structure. Silicone-jointed structural glass corners (no corner post) are standard practice and cost less than most buyers fear — ask to see one installed before you pay for steel.

Question 7

Are garden rooms always the cheap option?

Accepted Answer

Per square metre, yes — but site costs are fixed and they eat small projects alive. Garden room shell rates run £1,100–2,200/m² installed — roughly 40–50% below a linked glass box in the same region. Fixed site items do not shrink with the room: screw piles or a slab at £1,800–4,000, a 20 m armoured-cable trench at £1,200–2,500, drainage at £500–1,500 — £4,000–8,000 before the room arrives. Below about 15 m² the fixed items swallow the per-m² advantage; above 20 m² the garden room wins decisively — which is exactly why the calculator prices area first.

Question 8

Is a glass link cheap because it is small?

Accepted Answer

Small rooms price high per square metre precisely because they are small — the fixed costs have nowhere to hide. Fixed costs do not scale with length: structural calculations £800–1,500, thermal-break junctions at both buildings, drainage at both ends — £8,000–15,000 fixed before a pane arrives. A 3 × 2.4 m house-to-garage link in London lands at ≈ £22,000–30,000 — that is £2,800–4,200/m², above the glass box rate, not below it. It is still the cheapest way to join two buildings without building a full room between them — just budget it as a fixed-price item, not a per-m² guess.

Question 9

Can you insulate a glass roof at all?

Accepted Answer

Yes — three ways, and two of them keep the light. Triple-glazed structural roof units reach U 0.7–0.9 W/m²K for a 15–20% premium over the double-glazed version. A solid insulated roof panel hits U 0.15–0.2 with two rooflights set in — the warmest and cheapest route, at the cost of the full-glass ceiling. Double-glazed roof glass with warm-edge spacers lands at U ≈ 1.0–1.2 W/m²K — the common garden-room compromise when light is the priority.

System	London	UK regional	Australia
🪟 Structural glass box	£2,400–3,600	£1,900–2,900	A$3,200–4,800
🚪 Aluminium framed	£1,900–2,900	£1,500–2,300	A$2,400–3,800
🔗 Glass link	£2,800–4,200	£2,200–3,400	A$3,600–5,400
🌳 Garden room	£1,400–2,200	£1,100–1,800	A$1,800–3,000

Floor area	Budget tier	Balanced tier	Premium tier
10 m² (smallest sensible box)	£24,000	£30,000	£36,000
15 m² (kitchen bump-out)	£36,000	£45,000	£54,000
20 m² (classic glass box)	£48,000	£60,000	£72,000
30 m² (open-plan living)	£72,000	£90,000	£108,000
40 m² (whole-rear rebuild)	£96,000	£120,000	£144,000

❌ The myth	Glass rooms are iceboxes in winter
✓ The reality	Current-spec glazing loses ≈ 800 W on a 27 m² box — one radiator

Glass Extension Cost Calculator – Per m² Price Guide

Which project are you pricing? Follow the knots

Glass Extension Cost Calculator

Your budget: Structural glass box · London

Where the balanced-tier money goes

Glass Extension Cost Per m²: 2026 Rate Benchmarks

What Drives the Price of a Glass Box Extension

Glass specification

Door lines

Roof choice

Groundworks & drainage

Structure & corners

Access & region

Glass Link Between Buildings: Pricing Short Runs

Glass Garden Rooms Costs UK: The Standalone Build

Planning Permission and Building Rules: The Generic Picture

The Budget Formula (and a Copyable Version)

Glass Extension Cost Myths: Decision-Tree FAQ

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