A Velux (or flat roof window) loft conversion — using roof windows set flush within the existing roof slope — is the simplest, most affordable, and least disruptive loft conversion type. It is ideal for properties where the existing roof has adequate headroom, a suitable pitch, and the client's brief can be met within the existing roof envelope.
What Is a Velux Loft Conversion?
A Velux conversion (the term is commonly used generically for any roof window, though Velux is actually the dominant brand) adds roof windows to the existing roof slope rather than building a dormer extension. The roof structure remains essentially unchanged; the conversion involves:
- Strengthening the floor (new structural joists)
- Insulating the roof slope to Building Regulations standard
- Installing roof windows in cut openings in the existing roof covering
- Creating the loft access (staircase)
- Internal lining, plastering, and second fix
Velux vs Dormer: When Does a Velux Work?
A Velux conversion works well when:
- The existing ridge height provides adequate headroom (approx. 2.2m minimum)
- The pitch is 35° or more (steeper pitches give more headroom)
- The requirement is for a single usable bedroom or home office, not multiple rooms
- Budget is the primary consideration (Velux conversion is significantly cheaper than a dormer)
- Planning risk is a concern (Velux conversions are often permitted development where dormers require planning)
A Velux conversion may not work when:
- Headroom is inadequate across an acceptable floor area
- Multiple rooms are required
- North-facing rear roofs reduce light quality significantly
Permitted Development for Velux Conversions
Velux roof windows (set flush with the roof slope, not projecting dormers) are usually permitted development on both front and rear slopes, provided they:
- Do not protrude more than 150mm from the plane of the roof
- Do not exceed the height of the existing ridge
This means a Velux conversion may not require planning permission at all — a significant advantage.
Types of Velux Window
Centre-pivot (GGU/GGL): The standard Velux. The sash pivots from the centre for cleaning and ventilation. Most affordable. Available in standard sizes from 55×78cm to 114×118cm.
Top-hung (GPU/GPL): Hinge is at the top, providing a clearer view and better ventilation position. Popular for living areas.
Cabrio roof terrace window: A specialist product that opens to create a balcony-style terrace. Popular in urban London lofts but requires planning permission.
Fixed (GXU/GXL): Non-opening, for light only.
Electrically operated (integra/solar integra): Remote-controlled opening. Useful for high windows. Available with rain sensors.
Typical Costs in London
| Scope | Approximate cost |
|---|---|
| Basic Velux conversion (2 windows, 1 room, no en-suite) | £18,000–£30,000 |
| Velux conversion with en-suite | £28,000–£42,000 |
| Velux conversion with home office + en-suite | £35,000–£50,000 |
Costs include structural works, insulation, windows, plastering, staircase, and basic second fix. Excludes kitchen and luxury fittings.
rcbGroup
rcbGroup delivers Velux (roof window) loft conversions as part of the full range of loft conversion options — helping clients choose the right conversion type for their roof, headroom, and brief.