Loft Conversion Types Explained: Dormer vs Velux vs Mansard

What Is a Velux Loft Conversion?

Velux conversions (also called roof light conversions) are the simplest type. They involve installing roof windows, adding insulation, laying a floor, and fitting a staircase. No alterations to the roofline are needed.

Costs: £40,000-£55,000. Timeline: 4 to 6 weeks. Planning: Usually Permitted Development. Best for: lofts with good existing headroom (2.2m+ from joist to ridge).

What Is a Dormer Loft Conversion?

Dormer conversions project vertically from the roof slope, creating a boxy extension. This adds significant headroom and floor space. Rear dormers are most common; side dormers are less typical.

Costs: £45,000-£65,000. Timeline: 6 to 8 weeks. Planning: Often Permitted Development, but check restrictions. Best for creating full-height rooms with vertical windows.

What Is a Mansard Loft Conversion?

Mansard conversions alter the entire roof slope, creating a near vertical wall (typically 72 degrees) with a flat roof. This maximises internal space and is common in London terraces.

Costs: £55,000-£80,000+. Timeline: 8 to 12 weeks. Planning: Usually requires full planning permission. Best for maximising space in terraced houses, particularly Victorian and Edwardian properties.

What Is a Hip-to-Gable Conversion?

Hip-to-gable conversions replace a hipped (sloping) roof end with a vertical gable wall. This is common on 1930s semis and end of terrace properties. Often combined with a rear dormer.

Costs: £50,000-£70,000. Timeline: 6 to 10 weeks. Planning: Often Permitted Development. Best for semi-detached and end-of-terrace properties with hipped roofs.

Which Loft Conversion is Best?

The best type depends on your roof structure, budget, and space needs. Velux is ideal if you have good headroom and want a simple, affordable solution. Dormer offers the best balance of space and cost. Mansard maximises space but costs more and needs planning.

We provide free loft surveys to assess suitability and recommend the optimal conversion type for your property and budget.

What Are the Permitted Development Rules for Lofts?

Most loft conversions in England can be built under permitted development, which means no planning application, provided the extra roof space does not exceed 40 cubic metres for a terraced house or 50 cubic metres for a semi-detached or detached house. The roof extension cannot extend beyond the plane of the existing roof slope at the front, and materials should be similar in appearance to the existing house.

Permitted development rights for roof extensions of this kind do not apply in conservation areas and may be removed by an Article 4 Direction elsewhere, so check before you plan. A Lawful Development Certificate gives written confirmation that the work is lawful.

What Do Building Regulations Require in a Loft?

A loft conversion must meet Building Regulations even where planning permission is not needed. The main points are a structurally sound new floor, safe stairs with enough headroom, fire protection and escape routes, insulation to the current standard, and proper sound insulation between floors. Because the new room becomes a third storey, additional fire safety measures are required, usually including fire doors and interlinked smoke alarms.

If you share a party wall with a neighbour, you will usually also need to follow the Party Wall etc. Act 1996 and serve notice before work begins.

Next Steps

We can advise which type of loft conversion suits your roof and budget, then give you a fixed price in writing. Call 020 4569 0526 or use our contact page to arrange a visit.

Sources and Further Reading

Planning Portal, loft conversions: https://www.planningportal.co.uk/permission/common-projects/loft-conversion

Party walls: https://www.gov.uk/party-wall-etc-act-1996-guidance

Approved Documents (Building Regulations): https://www.gov.uk/government/collections/approved-documents