A roof window transforms a dark loft space into a bright, usable room. Whether you are converting a loft, replacing a dated skylight, or adding natural light to a top-floor extension, the roof window market — dominated but not limited to Velux — offers a wide range of options with different opening mechanisms, glazing specifications, and price points.
This guide covers the standard sizes available in the UK, what installation typically costs, when you need planning permission, and how to choose the right window for your roof.
Velux and Its Competitors
Velux is the dominant brand in the UK roof window market and their product codes have effectively become the shorthand for the entire category — much like “hoover” for vacuum cleaners. Other reputable manufacturers include FAKRO, RoofLITE, and DAKEA, all of which offer compatible sizing in the same standardised format and generally come in 10–25 % cheaper than equivalent Velux units for similar specification.
All major brands are available through builders’ merchants and roofing suppliers. Genuine Velux products carry a 10-year product guarantee; most competitors offer 5–10 years.
Standard Sizes
Velux and most competitors use an alphanumeric coding system where the letters denote width and the numbers denote height. The UK market centres on a handful of standard sizes that work with typical 600 mm rafter spacings.
| Velux Code | Width (mm) | Height (mm) | Typical Use |
|---|---|---|---|
| CK02 | 55 | 78 | Small utility rooms, stairwells |
| CK04 | 55 | 98 | Compact bedrooms, bathrooms |
| MK04 | 78 | 98 | Standard bedroom — most popular size |
| MK06 | 78 | 118 | Bedrooms, sitting rooms |
| MK08 | 78 | 140 | Living spaces, good egress |
| PK04 | 94 | 98 | Wide, lower-pitch installations |
| PK08 | 94 | 140 | Large bedrooms, loft sitting rooms |
| SK06 | 114 | 118 | Feature windows, south-facing orientations |
| SK08 | 114 | 140 | Maximum daylight — large spaces |
The MK04 (78×98 cm) is the workhorse of UK loft conversions, fitting neatly between standard 600 mm rafter centres with minimal structural intervention and meeting Building Regulations minimum area requirements for habitable rooms in most configurations.
For a habitable room, Building Regulations Approved Document F requires a minimum opening area of 1/20th of the floor area for ventilation, and Approved Document L specifies a maximum area of glazing relative to the floor to control heat loss (typically 25 % of floor area for rooflights without exceptional justification).
Opening Types
Centre-pivot: The most common type in the UK. The sash pivots on a horizontal axis at the centre of the frame, allowing the top of the window to swing inwards for cleaning and ventilation. Cleaning is easy from inside. Not suitable for pitches below around 15°.
Top-hung: The sash hinges at the top, opening outward like a traditional window. Gives a wider, unobstructed view and a lower internal projection than centre-pivot. Suitable for pitches as low as 15°. More expensive than equivalent centre-pivot.
Pivot-turn (combination): Operates as a centre-pivot but can also rotate 180° for external cleaning — useful on difficult-to-access roofs.
Fixed / non-opening (rooflight): No ventilation function — provides light only. Lower cost, simpler installation, and suitable for areas where ventilation comes from elsewhere. Not acceptable as the sole ventilation source for a habitable room.
Electrically operated: Available in any opening type. Electric operation is standard for windows positioned above 1.8 m that cannot be reached by hand; also available as an upgrade for lower positions. Remote control or app-controlled operation. Adds £150–£350 to the supply cost.
Glazing Options
All current Velux and equivalent products use double glazing as standard. Upgrades are available:
- Triple glazing: U-value around 0.7 W/m²K vs 1.2–1.4 W/m²K for standard double. Recommended for north-facing windows, cold climates, or high-performance envelopes. Adds £80–£200 to unit cost.
- Laminated inner pane: Adds safety and sound reduction. Required by some planning conditions in conservation areas; recommended where the window is accessible from outside.
- Solar control glazing: Limits solar gain — useful for south-facing roof windows in rooms prone to overheating. Reduces light transmission slightly.
- Privacy (obscured) glazing: For bathroom roof windows where overlooking is a concern, though most bathroom roof windows are too high to be overlooked anyway.
The minimum glazing standard for Building Regulations Part L (England) is a U-value of 1.6 W/m²K for roof windows in new build and major renovation; for existing dwellings the notional limit is 1.8 W/m²K when replacing on a like-for-like basis.
Flashings
A flashing kit seals the junction between the window frame and the roof covering — it is not optional and must be bought to match the roof tile profile and pitch. Velux and competitors offer specific flashing kits for:
- Plain tile (standard, low-profile)
- Interlocking concrete tile (deep profile)
- Natural slate
- Single-ply membrane / flat roof upstand
- Corrugated metal / fibre cement
The wrong flashing causes water ingress — always specify the flashing kit at the same time as the window unit. Flashing kits cost £80–£250 depending on size and tile type.
Cost to Supply and Fit
Supply Costs (mid-2026)
| Window | Velux (Supply Only) | Competitor Equiv. |
|---|---|---|
| MK04 centre-pivot, double glazing | £420–£520 | £280–£380 |
| MK06 centre-pivot, double glazing | £480–£600 | £320–£430 |
| PK08 top-hung, double glazing | £600–£780 | £400–£550 |
| SK08 centre-pivot, triple glazing | £750–£950 | £520–£700 |
| MK04 electric, double glazing | £650–£800 | £480–£620 |
Flashing kit: add £90–£220 per window.
Installation Costs
| Scenario | Cost Per Window |
|---|---|
| Like-for-like replacement (existing opening, matching tile) | £300–£450 |
| New installation (cut new opening, new flashing, make good) | £450–£700 |
| New installation requiring structural work (rafter trimming) | £600–£1,000 |
| Electric window with new cable from consumer unit | add £150–£250 |
A competent roofer and carpenter typically complete a straightforward replacement in half a day. New openings requiring rafter trimming (cutting and doubling up rafters to create a structural trimmer frame) add a day’s carpentry work.
Planning Permission
Roof windows are generally permitted development in England under the GPDO 2015 (as amended), subject to conditions:
- The window must not protrude more than 150 mm above the existing roof plane.
- On a roof slope that fronts a highway, the window must be flush with or not protrude above the roof plane.
- Not permitted on a listed building without listed building consent.
- Conservation areas: permitted development rights for roof windows are not removed simply by being in a conservation area, but the 150 mm protrusion rule applies to all elevations visible from the highway, not just the front.
In Scotland, Wales, and Northern Ireland, permitted development rules differ — check with your local planning authority before proceeding in these jurisdictions.
Building Regulations
Roof windows in habitable rooms trigger Building Regulations compliance for:
- Thermal performance (Part L): U-value requirements as above.
- Ventilation (Part F): The opening area must meet minimum ventilation requirements for the room. A background ventilation path (trickle vent) is required in most cases — Velux and competitors offer integrated trickle vents.
- Fire safety (Part B): If the window serves as a means of escape from an upper floor, it must have an unobstructed openable area of at least 0.33 m², with a minimum 450 mm clear width and height, and be no more than 1,100 mm above floor level. The MK06 (78×118 cm) typically meets this requirement when fully open on a centre-pivot.
- Structural: Cutting a new opening in a roof requires confirmation that the remaining rafters can carry the load without deflection. On most domestic roofs this is straightforward with proper trimming, but a structural engineer’s sign-off may be needed for large openings or unusual spans.
Building work involving new structural openings is notifiable to Building Control. For like-for-like replacement of an existing roof window, notification is generally not required but advice varies by local authority — check before starting.
