Summary
Roof lanterns are one of the highest-margin, highest-dispute items on a rear extension quote, because the price swings hugely on details that aren't visible until you're on site: whether the kerb already exists or has to be built from scratch, whether the opening spans far enough to trigger a structural engineer's calculation, and how many glazing and ventilation upgrades the customer wants layered on. A lantern quoted from a brochure size and a rough day-rate guess routinely comes in 30–50% under the real cost once the structural opening, thermal-bridge detailing and access equipment are properly accounted for.
The single biggest driver of price is span. Openings up to roughly 2.5m can usually be trimmed with doubled timber joists to a standard domestic detail; above that, Building Control will expect a structural calculation and often steel or engineered-timber support — a genuinely different scope of work, not just "a bigger lantern." The second driver is whether the job is new-build (kerb, structural opening and waterproofing all still to do) or a retrofit onto an existing, already-waterproofed upstand — the latter can be a one- or two-day fitting job, the former is a three- to five-day construction job with a lantern on top.
This guide gives 2026 UK pricing by size and scope, a worked cost breakdown showing how the quoted price is built from labour, materials, plant and margin, the add-ons that change the bill, regional variation, and what to itemise on the quote. For the installation technique — upstand construction, thermal bridging, flashing detail and the falls that keep water away from the lantern — see roof lantern installation. For the regulatory framework and terminology (lantern vs rooflight vs skylight), see roof lantern. For rooflights and skylights more broadly, see velux skylight pricing guide.
Key Facts
- Small lantern (up to 2m × 1.5m), kerb already built — £2,800–£4,500 supply and fit
- Medium lantern (2m × 2m to 3m × 2m), most common rear-extension size, kerb already built — £4,000–£7,000 supply and fit
- Large lantern (3m × 3m to 4m × 3m), span over 2.5m, needs structural calc — £7,000–£13,000+ supply and fit
- Bespoke/oversized (4m+ any dimension, or multi-panel/linked lanterns) — £13,000–£25,000+
- Kerb, structural opening and flat-roof waterproofing built as part of the job (not pre-existing) — add £1,200–£2,800 to any of the above
- Electric opening vent (per vent, motor + rain sensor) — £800–£1,500
- Solar control glass upgrade (g-value 0.3–0.5) — £400–£900
- Triple-glazing upgrade over standard double glazing — £1,200–£2,500
- Structural engineer's calculation (needed once span exceeds roughly 2.5m) — £350–£700
- Building control application/notification — £200–£600
- Scaffold or tower hire, if required for access — £400–£1,200
- Aluminium lantern day rate (2-person fitting crew) — £280–£350/day regional, £350–£450/day London/South East
- Fitting-only labour (kerb pre-built, straightforward access) — 1–2 days
- Full build (structural opening, kerb, membrane, lantern, internal reveal) — 3–5 days
- Whole-unit U-value target (new extension) — 1.2 W/m²K per Approved Document L 2021; manufacturer must supply a Uw certificate, not centre-pane
- VAT — standard 20%; roof lanterns forming part of a wider extension follow the same VAT treatment as the extension works
Quick Reference Table
Spending too long on quotes? squote turns a 2-minute voice recording into a professional quote.
Try squote free →| Scenario | Labour days | Materials & lantern unit | Typical quoted price | Notes |
|---|---|---|---|---|
| Small lantern, kerb already built | 1–1.5 | £1,600–£2,600 | £2,800–£4,500 | Straightforward fitting job |
| Medium lantern, kerb already built (most common) | 1.5–2.5 | £2,200–£3,800 | £4,000–£7,000 | 2×3m or 3×2m aluminium double-glazed |
| Large lantern, span >2.5m, kerb already built | 2.5–4 | £3,800–£7,000 | £7,000–£13,000+ | Structural calc, steel/engineered trimmers |
| Medium lantern, full build (kerb + opening + membrane) | 3.5–5 | £2,800–£4,800 | £5,500–£9,500 | Kerb and waterproofing built from scratch |
| Large lantern, full build, span >2.5m | 5–7 | £4,600–£8,500 | £9,500–£17,500+ | Full new structural opening plus large unit |
| Electric opening vent (per vent) | 0.25 | £600–£1,200 | £800–£1,500 | Motor, rain sensor, wiring to isolator |
| Solar control glass upgrade | — | £300–£700 | £400–£900 | South-facing kitchen extensions especially |
| Structural engineer's calculation | — | — | £350–£700 | Required once span exceeds ~2.5m |
| Scaffold/tower hire | — | — | £400–£1,200 | Height and duration dependent |
Detailed Guidance
How the quoted price is actually built
The figures above are what the customer pays, margin included. Here's how a specialist glazing/carpentry contractor gets there.
Worked example — 3m × 2m aluminium double-glazed lantern, span 3m (above the 2.5m structural threshold), fitted onto a new structural opening with kerb still to build:
| Cost element | Amount |
|---|---|
| Lantern unit (aluminium, double-glazed argon, low-E, solar control, Uw 1.2) | £2,200 |
| Upstand kerb materials (timber, PIR insulation, ply, membrane dressing) | £350 |
| Structural trimmers/timber for the opening, plus fixings | £280 |
| Structural engineer's calculation | £450 |
| Labour, 2-person crew, 3 days at £560/day (crew total) | £1,680 |
| Waste/skip | £120 |
| Tower hire (flat roof, moderate height) | £250 |
| Subtotal — cost | £5,330 |
| Margin at 25% | £1,332 |
| Quoted price to customer | £6,662 |
That £6,662 lands at the upper end of the medium bracket for a full build (£5,500–£9,500) — correctly positioned, because a 3m span forces the structural calc and steel/engineered trimmers even though the unit itself is a standard "medium" size. If the same lantern were dropped onto an already-built, already-waterproofed kerb (no structural work, no engineer's calc, no tower needed because scaffold access already exists from other extension trades), the labour drops to 2 days (£1,120), the kerb materials and trimmer costs disappear, and the subtotal falls to £2,200 (unit) + £120 (waste) = £2,320. At the same 25% margin that's a quoted price of £2,900 — below the medium "kerb already built" bracket of £4,000–£7,000, which flags that a 3m-span lantern genuinely belongs in the large bracket once you account for its structural calc, even on a fitting-only job. Re-running the calc with the structural engineer's fee (£450) added back in: subtotal £2,770, margin 25% = £693, quoted price £3,463 — still below £4,000. The honest read here is that a 3m span sits right at the boundary between "medium" and "large" brackets; quote it against the large bracket's lower end (£7,000+) once the full build scope and structural sign-off are included, and only price it as "medium fitting-only" if the span genuinely stays at or under 2.5m and no structural calc is needed.
Why span, not glass area, drives the structural cost
Customers compare lanterns by floor area covered, but Building Control and the structural calculation care about span — the distance a trimmer or beam has to carry without intermediate support. Up to roughly 2.5m, doubled timber trimmers (200×47 or 200×75, C24 grade) on a standard domestic joist deck usually suffice without an individual calculation. Beyond that, expect a structural engineer's calc as standard, and increasingly a steel ring beam or engineered timber (LVL/glulam) trimmer specified by the engineer rather than assumed by the fitter. This is the single most common place a quote goes wrong: a fitter prices a "3m lantern" using the labour and materials assumptions for a 2m lantern, because the glass looks similar in the brochure, and misses that the structural scope has changed entirely.
New build vs retrofit — the biggest swing in the quote
- Kerb already built (lantern is being fitted as the final trade onto a completed, waterproofed upstand — common where a roofer has already formed and membraned the kerb as part of the flat roof works): this is essentially a fitting-only job — position, mechanical fix, seal, verify drainage, internal trim. 1–2.5 days depending on size.
- Full build (the lantern contractor is also forming the structural opening, building the kerb, insulating it to avoid a thermal bridge, and dressing the membrane): 3–5 days, and the materials list grows substantially — timber, PIR, ply, membrane, fixings — on top of the lantern unit itself.
Always confirm on site (not from the client's description) which scenario applies before quoting. "The roof's already done" from a homeowner sometimes means the flat roof deck is down but the kerb and membrane upstand are not — a materially different, more expensive job than the one they think they're asking for.
Pricing the add-ons
- Electric opening vents add real cost per vent — motor, rain sensor, wiring back to an isolator or switch, and (on larger lanterns) a dedicated circuit. £800–£1,500 per vent is typical; a lantern with two opening vents can add £1,600–£3,000 to the base price.
- Solar control glass is worth flagging proactively on south-facing kitchen extensions — the alternative is a customer complaint about summer overheating six months after completion, and a retrofit blind or film costs more than specifying the glass correctly at the outset.
- Triple glazing rarely pays for itself in thermal performance on a domestic lantern once you're already meeting the 1.2 W/m²K target with double glazing and low-E coatings — price it as a customer preference (weight, sightlines, acoustic performance) rather than a compliance requirement.
Regional variation
Fitting crew day rates run £280–£350/day in most of the UK and £350–£450/day in London and the South East. Lantern unit pricing itself (aluminium, standard sizes) doesn't vary much regionally since it's largely a manufactured, delivered product — the regional swing comes almost entirely from labour and scaffold/access costs.
Red flags that push the price to the top of the band
- Span over 2.5m — structural calc and engineered support required as standard; don't quote it as a scaled-up "medium" job.
- No existing scaffold or safe roof access — a standalone tower or scaffold hire becomes a real line item, not a rounding error.
- South-facing orientation with no solar control specified by the customer — flag it before quoting; retrofitting shading later costs more than specifying correctly.
- Listed building, conservation area, or principal (front) elevation — Permitted Development doesn't apply on the principal elevation for rooflights; check planning status before booking the job, not after ordering the unit.
- Poor upstand condition on a retrofit job — if the existing kerb isn't level (±2mm target) or square (diagonals within 5mm), rework is needed before the lantern can go on, and this is easy to miss on a desk-based quote.
What to itemise on the quote
- Structural opening and support (or confirmation the opening is pre-formed and to what specification)
- Structural engineer's calculation, where applicable, and who is responsible for the Building Control submission
- Upstand kerb construction and insulation (or confirmation it's pre-built)
- Flat-roof membrane and flashing dress-up at the upstand
- Lantern unit — size, glazing spec (U-value, solar control, opening vents), frame finish
- Internal reveal/plastering
- Scaffold/access equipment
- Building Control notification fee
Frequently Asked Questions
Why did one quote come in at £4,500 and another at £8,000 for what looks like the same lantern?
Almost always because one quote assumes the kerb and structural opening are already built and the other assumes they need constructing from scratch, or because one span crosses the ~2.5m structural threshold and the other doesn't. Ask both contractors exactly what's included — same-size lanterns can be genuinely different jobs.
Is it cheaper to buy the lantern separately and just pay for fitting?
Sometimes, but it removes the single-point warranty most manufacturers offer for supply-and-fit by their approved network, and it puts the responsibility for correct sizing, Uw certification and any defects on the customer rather than the installer. For most domestic jobs the cost saving is modest against the loss of warranty cover and installer accountability.
Do I need planning permission for a roof lantern?
Usually no, since most lanterns sit on extensions that are themselves Permitted Development — but permission is needed if the property is listed, in a conservation area, the lantern overlooks a neighbour from an upper-floor flat roof, or it's on a principal (front) elevation, which Permitted Development excludes. Always confirm with the local planning authority before ordering, and price this check into the job timeline.
What's the cheapest way to add roof light without a full lantern?
A flat rooflight (a single or twin pane flush or near-flush with the roof plane) is typically 30–60% cheaper than a lantern of similar glazed area and simpler to install, because there's less frame, no pyramid structure, and often a smaller structural opening. See velux skylight pricing guide for pricing.
Regulations & Standards
Building Regulations Approved Document L (2021) — whole-unit U-value target 1.2 W/m²K for new extensions, 1.6 W/m²K for replacements; drives the glazing specification and cost
Building Regulations Approved Document F (2021) — ventilation; relevant when pricing opening vents as a compliance item rather than a preference
Building Regulations Approved Document K — overhead safety glazing (laminated or toughened); factored into the lantern unit spec, not usually an extra cost once correctly specified
Building Regulations Approved Document A — structure; the trigger for the structural engineer's calculation on wider spans
BS 6229:2018 — flat roofs with continuously supported flexible waterproof coverings; governs the membrane and falls pricing around the kerb
BS EN 14351-1 — windows and external pedestrian doorsets product standard (UKCA marking); the lantern unit must be certified against this
CDM Regulations 2015 — notifiable work-at-height requirements, relevant to scaffold/tower pricing
Approved Document L (2021) — UK Government
Approved Document F (2021) — UK Government
Approved Document K — UK Government
Planning Portal — roof alterations guidance — Permitted Development conditions
The Glass and Glazing Federation (GGF) — industry guidance on glazing and rooflight specification
roof lantern installation — installation technique: structural opening, kerb construction, flashing and thermal bridging in detail
roof lantern — regulatory framework, permitted development and glazing standards
velux skylight pricing guide — pricing for flat rooflights and pitched-roof rooflights as cheaper alternatives
flat roof falls and drainage — setting falls to move water away from the lantern upstand, priced separately on new-build kerbs