What Is a Velux Loft Conversion?
A Velux conversion (also called a roof-light or conservation roof-light conversion) keeps the existing roof structure largely intact. Light and ventilation are provided by Velux or equivalent roof lights fitted flush within the existing roof slope.
The floor area is limited to what is available within the existing roof void — you cannot extend beyond the existing footprint.
When Does a Velux Loft Conversion Work?
A Velux conversion works when:
- The existing loft has adequate head height (minimum 2.2m at ridge, typically, for a comfortable room)
- The roof pitch is steep enough to give usable floor area across the room
- The project budget is limited
- Planning constraints (conservation area, listed building) restrict addition of dormers
- Only a simple bedroom or study without bathroom is needed (limited plumbing)
Advantages of a Velux Conversion
- Lower cost than a dormer conversion
- Faster to build (typically 6–10 weeks)
- Least disruption to the existing roof structure
- Often does not require planning permission (roof-light only conversions are usually PD for rear-facing lights, subject to limits)
- Preserves the external appearance of the roof — important in Conservation Areas
Disadvantages
- Limited headroom — the sloping ceiling restricts usable floor area
- Cannot add space beyond the existing footprint
- Difficult to add a bathroom at the ridge line without a high roof pitch
- Feels more constrained than a dormer conversion
Typical Cost
£25,000–£45,000 (including staircase, first and second fix, basic decoration; excluding floor finishes and furniture)
What Is a Dormer Loft Conversion?
A dormer is a structural projection built out from the roof slope, creating a vertical wall and a horizontal or shallow-pitched roof section. This gives full-height (approximately 2.4m) headroom over a larger proportion of the floor area.
The most common type in London is the rear dormer, which runs across most or all of the rear roof slope, creating a substantial room. The hip-to-gable dormer (for semi-detached properties) extends the roof to a gable end, adding further floor area.
When Does a Dormer Conversion Work?
A dormer is appropriate when:
- You want a habitable bedroom with en-suite bathroom
- The existing roof pitch does not provide adequate headroom for a Velux conversion
- You want to maximise the usable floor area of the new room
- Planning permission is obtainable (rear dormers are PD on many properties outside Conservation Areas)
Advantages of a Dormer Conversion
- Significantly more usable floor area than a Velux conversion
- Full-height rooms — no sloping ceilings (except at the very eaves)
- Easily accommodates an en-suite bathroom
- Better natural light from vertical dormer windows than from roof lights
- Higher added value to the property
Disadvantages
- Higher cost
- Longer build programme (typically 10–16 weeks)
- Usually visible from the rear — may require planning permission
- Not possible in all Conservation Areas without special design consideration
Typical Cost
| Dormer Type | Estimated Cost |
|---|---|
| Rear dormer (standard) | £45,000–£75,000 |
| Hip-to-gable + rear dormer | £55,000–£90,000 |
| L-shaped dormer | £60,000–£90,000 |
| Full mansard conversion | £80,000–£140,000 |
(Figures include staircase, first and second fix, basic en-suite, decoration; excluding floor finishes and furniture. London market 2026.)
Planning Permission — Key Differences
| Conversion Type | Permitted Development? |
|---|---|
| Velux (rear, flat flush) | Usually yes (within PD criteria) |
| Rear dormer | Usually yes (outside Conservation Area, within PD criteria) |
| Hip-to-gable | Usually yes (semi-detached, outside Conservation Area) |
| Front dormer | No — requires planning |
| Any dormer in Conservation Area | No — requires planning |
| Mansard | Almost always requires planning |
Which Should You Choose?
| Scenario | Recommended |
|---|---|
| Tight budget (under £40,000) | Velux |
| Conservation Area, want minimal visual change | Velux |
| Want a proper bedroom + bathroom | Rear dormer |
| Semi-detached, want maximum space | Hip-to-gable + rear dormer |
| London terrace, want most space possible | Mansard |
| Victorian terrace with high-pitched roof | Rear dormer or mansard |
RCB Design & Build
RCB advises on the most appropriate loft conversion strategy for each property and client brief, taking into account head height, roof structure, planning context, and budget. We handle all types of loft conversion from permitted development Velux works through to full mansard schemes requiring planning permission.