Cost of a Loft Conversion in the UK (2025)
Loft conversion costs range from £25,000 for a basic Velux conversion to £80,000+ for a full mansard. The type of conversion and your property are the biggest factors.
Types of loft conversion
A Velux (or rooflight) conversion is the simplest and cheapest — no structural changes to the roof, just new floor, stairs, insulation, and Velux windows. A dormer conversion adds a box-like extension to the rear of the roof, creating more headroom and usable floor space. A hip-to-gable extends a side roof slope outwards. A mansard conversion involves rebuilding the rear roof slope nearly vertically — the most expensive option but creates the most space, common in London terraces.
Do I need planning permission?
Most loft conversions fall under Permitted Development, meaning no planning permission is required — provided they do not exceed 40m³ (semi/terrace) or 50m³ (detached) of additional roof space, and do not project above the existing roof line at the front. Mansard conversions almost always require planning permission. Your builder can advise what applies to your property.
How long does a loft conversion take?
A Velux conversion takes 4–6 weeks. A dormer conversion typically takes 8–12 weeks. Mansard conversions can take 4–6 months. You can usually remain in the property throughout, though there will be significant noise and dust during the structural phase.
Cost Breakdown
All prices are indicative UK averages for 2025. Actual costs vary by location, specification, and contractor. London and South East rates are typically 20–35% higher.
| Item | Low | High |
|---|---|---|
| Velux / rooflight conversion | £25,000 | £40,000 |
| Dormer conversion (single) | £35,000 | £55,000 |
| Hip-to-gable conversion | £40,000 | £65,000 |
| Mansard conversion | £55,000 | £85,000 |
| En suite addition (loft) | £5,000 | £12,000 |
| Staircase (loft-specific) | £2,000 | £5,000 |
What Affects the Cost?
- ✓Type of conversion — Velux vs dormer vs mansard
- ✓Whether an en suite bathroom is included
- ✓Quality of finishes specified
- ✓Existing roof structure and condition
- ✓Location — London conversions typically cost 30–50% more
- ✓Whether structural engineer fees are included in the quote
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Post a Job — It's FreeFrequently Asked Questions
Does a loft conversion add value?
Yes — a well-executed loft conversion typically adds 15–20% to property value. Adding a bedroom and en suite to a 3-bed home can add £40,000–£80,000 in value depending on the area.
Does a loft conversion need Building Regulations approval?
Yes — all loft conversions require Building Regulations approval regardless of planning permission status. This covers structural, fire safety, insulation, and electrical requirements. Your builder will manage this process.
What headroom do I need for a loft conversion?
You need a minimum of 2.2m of headroom at the ridge (highest point) for a practical loft conversion. Measure from the top of the existing ceiling joists to the underside of the ridge beam. Your builder can assess this for free.
Can all lofts be converted?
Most pitched-roof lofts can be converted, but some have roof structures (W-truss roofs common in 1960s–80s homes) that make conversion more complex and expensive. A builder or structural engineer can assess your specific loft.