A loft conversion is one of the most cost-effective ways to add space to your home. By converting unused roof space into a bedroom, office, or bathroom, you can significantly increase your property’s value and living area without extending your building’s footprint.
Can My Loft Be Converted?
Most lofts can be converted, but some are easier and more cost-effective than others. The key factors are:
Head Height
The most important measurement is the existing head height. You need at least 2.2 metres from the top of the ceiling joists to the underside of the ridge beam. Anything less than this makes a conversion challenging, though raising the ridge or lowering the ceiling below may be options.
Roof Structure
Traditional ‘cut roof’ construction with rafters and purlins is easier to convert than modern trussed roofs. Trussed roofs have W-shaped timber frames that occupy the loft space and require more structural work to open up the area.
Pitch Angle
Steeper roof pitches provide more usable floor space. A pitch of 30 degrees or more is ideal. Shallower pitches may require a dormer to create adequate head height.
Types of Loft Conversion
Velux or Roof Light Conversion
The simplest and most economical option, involving minimal structural alterations. The existing roof structure is retained and roof windows (such as Velux) are installed.
Approximate Cost: £20,000 – £40,000
Best for: Roofs with good existing head height
Pros: Quickest and cheapest, often permitted development
Cons: Limited floor space with full standing height
Rear Dormer Conversion
A flat-roofed box dormer extending from the rear roof slope. This is the most popular option in London and Surrey as it maximises floor space.
Approximate Cost: £40,000 – £65,000
Best for: Terraced and semi-detached houses
Pros: Significantly increased head height and floor area
Cons: Visible from the rear, may need planning permission
L-Shaped Dormer
Combines a rear dormer with a side dormer to create an L-shaped extension to the roof. Particularly suited to Victorian and Edwardian houses with a rear addition.
Cost: £50,000 – £80,000
Best for: Houses with rear outriggers
Pros: Maximum space, works with period property layouts
Cons: More complex, usually requires planning permission
Hip-to-Gable Conversion
For houses with hipped (sloping) ends to the roof, this extends the hip to form a vertical gable wall, significantly increasing the loft space.
Approximate Cost: £45,000 – £75,000
Best for: Semi-detached and detached houses with hipped roofs
Pros: Substantial additional floor space
Cons: Changes external appearance, planning permission usually required
Mansard Conversion
The most extensive option, replacing or modifying the entire roof structure with a new mansard roof. The near-vertical sides maximise internal space.
Approximate Cost: £65,000 – £100,000+
Best for: Properties where maximum space is the priority
Pros: Creates the most usable floor area
Cons: Most expensive, always requires planning permission
Planning Permission and Building Regulations
Permitted Development
Many loft conversions fall under permitted development rights, meaning you don’t need planning permission. The key limits are:
- Volume: Up to 40 cubic metres for terraced houses, 50 cubic metres for detached and semi-detached
- No extension above the existing ridge height
- Materials similar in appearance to the existing house
- No dormer on a roof slope facing a highway
- Side-facing windows must be obscure-glazed and non-opening below 1.7m
However, permitted development rights may be removed in conservation areas, on listed buildings, or by Article 4 directions.
Building Regulations
All loft conversions require building regulations approval, covering:
- Structural stability: Floor joists, beams, and load paths
- Fire safety: Protected staircase, fire doors, smoke alarms, escape windows
- Staircase design: Safe dimensions and headroom
- Sound insulation: Between the loft and rooms below
- Thermal insulation: Meeting current energy efficiency standards
Loft Conversion Costs
Costs vary significantly based on the type of conversion and your location:
| Conversion Type | UK Average | London/Surrey |
|---|---|---|
| Velux/roof light | £20k – £35k | £30k – £45k |
| Rear dormer | £40k – £55k | £50k – £70k |
| L-shaped dormer | £50k – £70k | £65k – £90k |
| Hip-to-gable | £45k – £65k | £55k – £80k |
| Mansard | £65k – £85k | £80k – £120k |
These figures include construction but typically exclude professional fees, which add 10-15%.
How Long Does a Loft Conversion Take?
Timescales depend on the complexity of your project:
- Velux conversion: 3-4 weeks on site
- Rear dormer: 5-7 weeks on site
- L-shaped dormer: 6-8 weeks on site
- Hip-to-gable: 6-8 weeks on site
- Mansard: 8-12 weeks on site
Add 8-12 weeks before construction for design, approvals, and contractor procurement. You can learn more about our design process to understand each stage.
Does a Loft Conversion Add Value?
A well-designed loft conversion typically adds more value than it costs. Industry estimates suggest:
- Adding a bedroom can increase property value by 10-15%
- Adding a bedroom with en-suite can add 20-25%
- ROI is typically 50-75% above the construction cost
In high-value areas like London and Surrey, where space is at a premium, the value added can be even greater.
Getting Started
If you’re considering a loft conversion, the first step is a feasibility assessment. We’ll visit your property, measure the loft space, assess the roof structure, and advise on the best conversion type for your needs and budget.
Contact us for a consultation. We specialise in loft conversions across London and Surrey, from simple Velux installations to complex mansard transformations.
We work with homeowners in Wimbledon, Wandsworth, Clapham, Battersea, Fulham, Kingston, Richmond, and Guildford. View our portfolio to see examples of our work.