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Velux roof windows or dormer windows — which to fit, what each costs, and how each changes the look of the loft.
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Costs, timelines, pitfalls and the right questions to ask before any loft conversions starts on loft Conversion Window Options.
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Hiring a loft conversions, without the regret.
Four moves that separate a smooth job from a nightmare.
Wrong flashing for tile
EDW for concrete tile, EDL for slate. Wrong flashing leaks within months.
Inadequate U-value
Standard Velux marginally compliant. Specify low-U-value premium for better thermal.
Condensation in winter
Cheap windows develop condensation. Trickle vent and quality glazing essential.
No escape window
Building Regs require escape route. Not all small Velux windows compliant.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and a typical week-by-week schedule.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Velux MK04 (medium) fitted78 × 98cm
£1.2k–£2k
Velux MK06 (large) fitted78 × 118cm
£1.5k–£2.5k
Velux Integra electricApp-controlled
£2k–£3.5k
Dormer window (single)Plus dormer construction
£800–£2.5k
Dormer with French doorsBalcony option
£1.5k–£4.5k
Sun tunnelFor dark areas
£500–£1.4k
!
Quote spread is typically ± 18% — always get 3 quotes.
Timeline
Dormer loft conversion · week by week
Typical
6 phases · 10 wk
6 phases · 10 wk
W1
W2
W3
W4
W5
W6
W7
W8
W9
W10
Structural + design
Strip + steels in
Dormer build + roof
1st + 2nd fix
Plaster + finishes
Snags + handover
!
Schedule slips on dependencies — pad each phase by 10–20% for real-world delays.
At a glance
Infographic · VideoThe Loft Conversions briefing.
Infographic
Loft Conversions quick-view
Video guide
Loft Conversions explained
Loft conversion windows are crucial for light, ventilation, and aesthetic. Velux roof windows or dormer windows are the two main options. Expect to pay £800–£3,500 per window fitted.
What Does the Work Involve?
Velux: integrated into roof slope, vertical light from above. Sizes from 78 × 98cm up to large 78 × 140cm. Dormer windows: vertical, like normal house windows. Project out from roof. Cleaner aesthetic, better headroom underneath. Mix is common.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| Velux MK04 (medium) fitted | 1,200 | 2,000 | 78 × 98cm |
| Velux MK06 (large) fitted | 1,500 | 2,500 | 78 × 118cm |
| Velux Integra electric | 2,000 | 3,500 | App-controlled |
| Dormer window (single) | 800 | 2,500 | Plus dormer construction |
| Dormer with French doors | 1,500 | 4,500 | Balcony option |
| Sun tunnel | 500 | 1,400 | For dark areas |
| Roof terrace doors (Velux Cabrio) | 3,500 | 8,500 | Opens into terrace |
How Long Does It Take?
Velux install: 1 day per window. Dormer construction (part of loft conversion): 1-2 weeks. Often during overall conversion.
DIY or Professional?
Velux retrofit: Velux Approved Installer. Dormer windows: part of full loft conversion by builder.
Choosing the Right Tradesperson
Velux Approved Installer for warranty. Loft conversion specialist for dormer windows. Brands: Velux (premium), Fakro (cheaper alternative).
UK Regulations
Part L: U-value 1.4 W/m²K or better. Part B: escape window in habitable rooms (clear opening 450mm minimum). Part F: ventilation. Building Regs notification.
Common Problems
- Wrong flashing for tile: EDW for concrete tile, EDL for slate. Wrong flashing leaks within months.
- Inadequate U-value: Standard Velux marginally compliant. Specify low-U-value premium for better thermal.
- Condensation in winter: Cheap windows develop condensation. Trickle vent and quality glazing essential.
- No escape window: Building Regs require escape route. Not all small Velux windows compliant.
Frequently Asked Questions
Velux or dormer window?
Velux: cheaper, less space lost, smaller install. Dormer: more headroom underneath, traditional window aesthetic, more cost.
How many windows for loft?
Typical loft bedroom: 2-3 Velux. Living space or large room: 3-4. Open-plan: 4-6.
Manual or electric?
Manual: cheapest, hand-cranked. Solar-electric: self-powered, app-controlled, weather-sensitive. Integra (mains): full smart-home integration.
Sun tunnel — when useful?
For dark internal areas (corridors, en-suites, walk-in wardrobes) without direct roof access. Brings light from roof to ceiling.
Escape window mandatory?
Yes — Part B requires clear opening 450mm minimum in habitable rooms. Larger Velux windows compliant; small ones not.
Velux Cabrio (roof terrace)?
Premium feature — Velux opens out forming a small balcony. £3,500-£8,500. Striking architectural feature.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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