Bespoke Aluminium Rooflights: The £10,000 Glass Box Above Your Kitchen

A 3-metre pane of triple-glazed glass costs roughly ten times what a Velux does, takes three times as long to arrive, and ships from a factory that doesn't install it. Get the upstand wrong and the specialist supplier's crew will turn around at the kerb and leave you with a rescheduling fee. Miss the lead time by three weeks and your builder sits idle waiting for glass to turn up. This is the glazing choice that catches project-managing homeowners out more than any other line on the spec.

Bespoke large-format aluminium rooflights are what sits above the island in every flat-roof kitchen extension you've seen on Instagram. Done right, they are the single most effective thing you can do to make a modest extension feel expensive. Done badly, they leak, they ice up internally on cold mornings, and they sag into the ceiling because nobody told the builder the kerb had to carry 180kg of glass.

Below: what they are, when they're worth the money over a pair of Velux windows, what a kerb actually has to do, how to order one without getting the measurements wrong, and what the whole thing costs.

What it is and what it's for

A bespoke aluminium rooflight is a made-to-measure fixed pane of insulated glass set into a thermally-broken aluminium frame, installed onto a flat or very-low-pitch roof. The supplier builds it to your exact opening size, ships it as a single sealed unit, and (on units over a certain size) sends their own crew to lift it into place. Fixed means it doesn't open. No hinges, no vents, no motor. Just a frameless view of sky.

The entire market exists because standard Velux-style roof windows max out at around 1.4m x 1.4m and live on pitched roofs. If you want a 3-metre span of glass over a flat-roof dining area, there is no off-the-shelf product that does it. A specialist fabricates one.

The technical term you'll see on supplier websites is "structural rooflight" or "flat glass rooflight". In practice the words "rooflight", "skylight" and "roof light" are used interchangeably. The distinction that matters is frame material: aluminium frames (with a polyamide thermal break) are the standard spec, because they can be powder-coated any RAL colour, they don't rot, and the thermal break stops condensation forming on the internal frame on cold nights.

This is the glass above the kitchen island. It's the reason your architect drew a 4m x 2.4m aperture on the roof plan and sent you to EOS for a quote.

Types and variants

Opening type

Three flavours, only one of which is common on domestic extensions.

Fixed. One sealed pane, no opening mechanism. Cheapest, simplest, most reliable. This is what the vast majority of extensions get. On a flat roof you cannot reach the rooflight to operate it anyway, so an opening mechanism just adds cost and failure points. Forum consensus on BuildHub is blunt: nobody who specified an opening flat-roof rooflight ever opens it.

Hinged/electric opening. A hinged sash with an electric motor and rain sensor. Roughly double the cost of an equivalent fixed unit. Justified if the rooflight is over a stairwell acting as a smoke vent, or if it's your only route for natural ventilation and you don't have MVHR. Rarely a good buy on a single-storey flat-roof kitchen.

Walk-on glass. Structural floor glass for basement light-wells or roof terraces. Different product, different price tier (indicative £2,500-£5,000+ per m² installed, confirm with supplier), needs a structural engineer sign-off because you're spanning a load-bearing aperture with glass people will stand on. Not what most kitchen extensions use.

For the standard flat-roof kitchen-diner extension, specify fixed. The cost saving pays for better glass.

Frame finish

Standard frame colours are powder-coated anthracite grey (RAL 7016) or matt black (RAL 9005). These are stocked by most fabricators and don't carry a bespoke-RAL surcharge. Any other RAL colour can be specified for a fee (indicative £150-£350 per unit, confirm with supplier).

Internally, the frame shows as a thin dark band around the glass (typically 40-80mm wide depending on product). The premium "frameless" spec pushes the frame outside the line of sight from below by stepping the glass down into the kerb, so when you stand underneath you see nothing but sky. Whether this premium is worth it is personal. Most people, once the rooflight is installed, stop noticing the frame.

Glass build-up

This is where suppliers separate themselves.

• Double-glazed argon-filled. Entry spec. Whole-unit U-value around 1.4-1.5 W/m²K. Fine for most extensions.
• Triple-glazed. Whole-unit U-values of 1.0-1.2 W/m²K. Better thermal performance, better acoustic performance (useful under flight paths), noticeably more expensive glass unit. Recommended for larger panes where heat loss is a bigger absolute number.
• Solar control coating. A metallic oxide layer on the glass that rejects solar infrared. Drops the g-value (solar heat gain) from around 0.6 to 0.3-0.4. Essential on south-facing rooflights or your kitchen will cook in July. Approved Document O now requires you to prove overheating mitigation on new dwellings, and solar control glass is the easiest way to do it on a rooflight.
• Self-cleaning. A hydrophilic coating (Pilkington Activ and similar) that lets rain sheet off the glass taking dust with it. Worth specifying on any rooflight you cannot reach with a window pole from the ground. Does not mean the rooflight never needs cleaning. It means it stays acceptable for two years instead of six months.
• Laminated inner pane. Laminated glass (two layers bonded with a PVB interlayer) is mandatory on overhead glazing in domestic settings. If a pane fractures, the interlayer holds the shards together instead of dropping them onto the dining table. Any reputable supplier will include this automatically. If a quote doesn't mention laminated glass, ask.
• Fire-resistance glass. Pilkington Pyrostop (EI30) or Pyrodur (EW30). Required under Approved Document B if the rooflight sits over an escape route. Adds significant cost and weight. Covered in the building regulations section below.

Supplier tier

Specialist UK suppliers worth naming for comparison (no endorsement):

• EOS Rooflights (Sidcup, Kent). Stock ranges with short lead times plus made-to-measure; strong customer reputation.
• Glazing Vision. Premium architectural glazing, full design service.
• The Rooflight Company. Conservation and modern ranges; established brand.
• Roof Maker. Strong mid-market spec, 20-year unit warranty on some lines.
• Korniche. Flat glass and lantern ranges, maximum pane 3m x 1.7m.
• Whitesales. Large fabricator supplying both stock and bespoke.
• IQ Glass. Architectural-specification supplier; premium pricing, premium build.
• HBD Systems / EOS-adjacent fabricators. Used on many UK kitchen extensions; competent bespoke fabrication.

Get three quotes from different tier suppliers and the range will surprise you. A 3m x 2m fixed aluminium unit can be £6,500 from one fabricator and £11,000 from another for what looks on paper like the same product. The spec differences are real (thermal break depth, seal system, glass build-up, frame extrusion wall thickness), but the pricing is far less standardised than you'd expect for architectural components.

Bespoke aluminium vs Velux-style: which do you need

This is the decision most extension projects have to make. The comparison isn't subtle.

Rule of thumb: below about 2m x 1.2m, specify two Velux-style roof windows on a pitched roof or a standard off-the-shelf flat rooflight. Above that, you're into bespoke territory whether you want to be or not. There's also a middle ground where the question becomes architectural rather than structural: if the design calls for a single uninterrupted pane over a 3m kitchen island, only a bespoke unit will do it. If two smaller units are acceptable, you can halve the cost and reduce the failure consequence (one seal failure is half the replacement cost).

Forum users on BuildHub consistently suggest pairing 2m x 1m standard units over specifying a single 3m x 2m bespoke pane where the design allows. Smaller units have shorter lead times, lower crane costs, and if a seal fails in 12 years, you replace one unit instead of a £10,000 piece of glass.

How to work with it

The kerb is your problem, not the supplier's

This is the single most important thing on the page. The specialist supplier fabricates the rooflight. They do not build the kerb. The main contractor builds the kerb, to a specification the supplier gives them, ready and waterproofed before the delivery date.

If the kerb is wrong when the supplier's crew arrives, one of three things happens. They refuse the install and charge you a wasted journey fee (typically £400-£800). They install it anyway over a non-compliant kerb and warranty is voided. Or they reschedule, and you lose your slot for another 3-6 weeks.

A correct kerb for a flat-roof aluminium rooflight looks like this:

• Framing: 95mm x 45mm CLS studs (or 63mm x 38mm treated CLS on smaller units), fixed to the roof joists below.
• Insulation: 70mm PIR board between the studs, flush with the external face.
• External sheathing: 9mm OSB3 board nailed to the outside of the kerb, continuous with the roof deck.
• Height: minimum 150mm exposed above the finished roof covering. Not the deck. The finished covering. Driving rain and wind uplift can push water higher than you'd think, and a 100mm kerb is the number-one cause of leaks.
• Fall: the overall flat roof must achieve a minimum fall of 1:80 to comply with BS 6229 (design to 1:40 to guarantee it after deflection). Some specialist rooflight suppliers (Sunsquare and others) recommend a steeper minimum of 3 degrees specifically across the glass surface to avoid surface ponding and water-marking. Confirm the required fall with your chosen rooflight supplier before the kerb is set out, because correcting it after the EPDM is on is painful.
• Internal VCL: vapour control layer on the warm side, continuous with the warm-roof VCL. Building control will check this before plasterboard goes up.
• External waterproofing: the EPDM membrane (or GRP) must lap 150mm up all four kerb faces before the rooflight arrives. The supplier lands the rooflight on top of the waterproofed kerb and seals around the frame with butyl tape and a compression fixing. They do not waterproof the kerb itself.

Delivery and installation

Large bespoke rooflights weigh 40-80 kg per square metre of glass. A 3m x 2m triple-glazed unit can therefore weigh 300-400kg. Anything over 150kg requires mechanical lifting. That means one of:

• Spider crane. A compact tracked crane that fits through a narrow doorway and sets up on the roof or in the garden. Typical hire £500-£900/day.
• Mobile crane. A larger unit operated from the street, with an extended boom over the property. £1,200-£2,500/day plus potential road closure fee.
• Glass suckers and manual lift crew. On smaller units (under about 2m x 1.5m / under 150kg), 4-6 fitters with vacuum suction cups can hand-lift the unit onto the kerb.

The specialist supplier's install quote usually bundles crane hire and glass-handling equipment. Read the quote carefully: some suppliers (IQ Glass, for example) exclude crane from their initial quote on the basis that it's often shared with other lifting operations on the same site. If you're co-ordinating with a structural steel lift on the same day, one crane hire can cover both operations and save a crane day's hire.

Access matters. A narrow side return, a garden accessible only through the house, or overhead cables on the street can all kill a spider-crane plan. Your supplier should do a site survey before confirming the install date. If they haven't asked about access, ask them why.

Sequencing on the build programme

A rooflight belongs near the end of the warm-roof build-up. The rough sequence:

1. Flat roof joists and deck in place, structural engineer sign-off on the opening.
2. Kerb framed and OSB-sheathed by the main contractor.
3. Warm-roof insulation dressed over the deck and lapped up the kerb faces.
4. EPDM or GRP membrane installed and dressed 150mm up all four kerb faces.
5. Specialist supplier arrives, lifts rooflight onto kerb, seals and fixes.
6. Internal VCL, insulation reveal, plasterboard reveal, plaster finish.

The hold point is step 4. The rooflight cannot arrive until the kerb is fully waterproofed. On a typical extension programme, that's 3-5 weeks from the flat roof deck going on. Back-calculate from there and you get the order date: if the lead time is 12 weeks, you need to place the order at structural-engineer sign-off, not after the kerb is built. Every week of delay on placing the order is a week your builder sits on an open roof.

How much do you need

Rooflights aren't a material you buy in bulk. You order a specific aperture. The sizing question is different. It's about how much glass to put in the roof.

Industry guidance from the Homebuilding & Renovating cost guides and from Approved Document O converges on rooflight area of 5-15% of the floor area below. Less than 5% and the room feels dark even with rooflights. More than 15% and you're into overheating territory, Part O compliance becomes difficult, and your glazing budget dominates the build.

Worked example: a 30m² kitchen-diner extension. 5-15% target = 1.5-4.5m² of rooflight. That's typically either two mid-size units (e.g. 1.5m x 1.2m each = 3.6m² total) or one large unit (e.g. 2.5m x 1.5m = 3.75m²). Two smaller units gives better light distribution; one large unit gives the architectural statement.

On Part O: total glazed area of the dwelling (including windows) cannot exceed the percentages in Table 1.1/1.2 of the approved document unless you demonstrate equivalent carbon performance through an overheating calculation. A large rooflight on a south-facing roof will often push you over the line without solar-control glass.

Cost and where to buy

There is no "average" cost because bespoke means bespoke. What there is, is size bands. Across EOS, Roof Maker, Whitesales, Korniche and mid-tier specialists surveyed April 2026, the pricing landscape for a fixed double/triple-glazed aluminium rooflight looks like this:

Specialist install labour (2-3 fitters plus lifting equipment): £800–£2,000 per unit. This covers the supplier's crew on the day; crane hire is sometimes bundled and sometimes separate, so check the quote.

Kerb construction by the main contractor: typically £300–£800 per opening in labour and materials. This is an indicative range based on a day's carpentry plus timber, OSB, PIR and membranes; it's not a market price, so treat it as a budget line rather than a quote.

A documented 30m² kitchen extension in Oxfordshire (2022 prices) spent £11,748 net on two bespoke fixed aluminium units, one at 3,525 x 2,380mm and one at 2,325 x 2,380mm, supplied by a specialist fabricator with 30-minute fire-resistance glazing (Pilkington Thermal K) on both units because they sat above an escape route to the rear garden. That's a real anchor for the upper size band.

Stick that alongside a standard Velux and the magnitude of the decision becomes obvious:

A pitched roof with four Velux windows would cost around £3,000–£7,000 fully installed (indicative all-in, see Velux-style roof windows for the line-by-line breakdown). A single bespoke rooflight of equivalent glazed area is in the £8,000-£15,000 range plus crane and kerb. The delta buys you: one uninterrupted pane, flat-roof compatibility, and architectural drama. That's it. Everything else the Velux does better or cheaper.

Specialist suppliers vs general merchants

Specialist glazing fabricators (EOS, Roof Maker, Glazing Vision, Korniche, IQ Glass, The Rooflight Company, Whitesales) are where this product comes from. They quote on drawings, manufacture to order, and arrange installation. There is no builders-merchant equivalent for this product. Travis Perkins does not sell a 3m x 2m bespoke rooflight.

General roofing merchants like Roofing Superstore carry stock-size rooflights (typically from Whitesales or similar) up to about 2m x 1m. Useful if your design allows an off-the-shelf size. Anything larger or with special glass spec goes direct to the fabricator.

Architectural glazing companies (the premium tier) bundle the design advice into the quote. Expect to pay a 20-40% premium over a mid-market fabricator for the same physical product, but get structural calculations, site survey, crane co-ordination, and longer warranties included. Worth it on a trophy project; overkill on a standard extension.

Building regulations

Four Approved Documents touch this product. Your building control officer will look at all four.

Part L (Thermal). The limiting whole-unit U-value for rooflights under Approved Document L 2022 is 2.2 W/m²K, measured in horizontal position. Any reputable bespoke aluminium unit beats this comfortably. The notional target for new-dwelling compliance is around 1.6 W/m²K, which most triple-glazed specs hit. The practical implication: don't let a supplier quote you a centre-pane U-value; insist on whole-unit. Some marketing materials quote the centre pane, which ignores the frame and gives a figure 20-30% better than the unit will actually achieve in use. BCOs can exercise some flexibility where the overall roof build-up compensates for a weaker rooflight, but don't plan around it.

Part B (Fire). If the rooflight sits over an escape route (a protected stair, or the internal circulation path from a bedroom to the final exit door), Approved Document B requires fire-resistance glazing. Minimum spec is E30 (integrity only, holds flame back for 30 minutes) for escape-route protection; EI30 (integrity + insulation) if there's no area restriction on the glazing. Pilkington Pyrostop achieves EI30; Pyrodur achieves EW30 (radiation reduction, not full insulation). Fire glass is thicker, heavier, and more expensive, so budget an extra 20-40% on the glass cost. This is the single requirement most homeowners miss.

Part O (Overheating). New dwellings (including extensions that create new bedrooms or substantially alter habitable rooms) must comply with Approved Document O for overheating. Large rooflights on south-facing roofs are a red flag for compliance. The easy answers are solar-control glass, external shading (brise-soleil, fixed louvres over the rooflight), or reducing the glazed area. Internal blinds do not count as shading for Part O purposes.

Part K (Safety glazing). Laminated inner pane on overhead glazing is mandatory. Any competent supplier includes this by default. If a quote doesn't mention laminated glass, question it before placing the order.

Get a pre-construction conversation with your BCO at the drawing stage. Show them the rooflight spec sheet. Confirm which parts apply to your specific design. Do not rely on the supplier to tell you what regs apply. They sell rooflights, they don't approve your building.

Where you'll need this

• Roof covering - flat-roof extensions where a bespoke rooflight is specified as part of the roof build-up
• Roof structure - large rooflight apertures require structural engineer sign-off on the opening
• Building control inspection: structure - fire glass, thermal performance, and laminated glazing are all checked at structure stage

These rooflights appear on any extension or renovation with a flat-roof element and an architectural glazing requirement. The structural, thermal and waterproofing detailing described above applies wherever a large fixed pane of glass is being dropped into a flat or very-low-pitch roof.

Common mistakes

Ordering before the structural engineer signs off the opening. Changing the aperture size after the fabrication deposit has cleared is painful. Wait for SE calculations, then order.

Assuming the supplier includes the kerb. They don't. The main contractor builds the kerb. The supplier fabricates and lands the rooflight on a kerb that's already waterproofed.

Undersized kerb. A 100mm kerb is a future leak. 150mm minimum exposed above the finished roof covering, always.

No fire glass over an escape route. Approved Document B is not optional. A BCO can require a rooflight to be replaced with fire-rated glazing if you specified the wrong spec. That's a five-figure mistake.

Centre-pane U-value confusion. If a supplier quotes a U-value without saying "whole unit" or "Uw", assume it's centre-pane and add roughly 25% to the figure before checking Part L compliance.

Late crane booking. Crane hire in city centres requires 2-3 weeks' notice, especially if a road closure is involved. Book the crane the moment the rooflight delivery date is confirmed.

Assuming Velux-style lead times. Eight to sixteen weeks, not one to two. A homeowner used to ordering Velux windows off the shelf can easily lose a month of programme by under-ordering the lead time on a bespoke unit.

Internal vs external measurement confusion. Written confirmation from the supplier, in the email chain before the deposit is paid, stating the exact dimension convention used. Not a verbal confirmation. Not a datasheet that could be interpreted two ways.

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