Overview
A loft extension can add valuable space, but the right option depends on the roof shape, planning position, budget and structural design. The Party Wall etc. Act 1996 may also apply, particularly where steel beams are cut into a party wall or the party wall is raised.
The cost figures below are broad guide figures only. Actual costs depend on location, specification and market conditions.
1. Simple dormer extension
A simple rear dormer adds headroom by extending the rear roof slope, often creating a box-shaped projection.
Typical guide cost: GBP30,000 to GBP40,000.
Typical duration: 3 to 4 months.
Planning: often possible under permitted development, subject to limits.
This is usually one of the most cost-effective options. It can create a bedroom and sometimes an en suite, although it may be less attractive externally than other options.
2. L-shaped dormer extension
An L-shaped dormer is common on Victorian properties with a rear outrigger. It can create significantly more floor area than a simple dormer.
Typical guide cost: GBP50,000 to GBP80,000.
Typical duration: 3 to 4 months.
Planning: often possible under permitted development, subject to limits.
This option can provide a larger bedroom, bathroom or more flexible layout.
3. Mansard loft extension
A mansard usually involves a more substantial alteration to the roof shape and may involve raising or altering party walls.
Typical guide cost: GBP35,000 to GBP75,000.
Typical duration: 4 to 5 months.
Planning: usually requires planning permission.
A mansard can be more attractive and provide better head height, but it is usually more complex.
4. Hip-to-gable conversion
A hip-to-gable conversion changes a sloping side roof into a vertical gable wall. It is common on end-of-terrace, semi-detached and detached houses.
Typical guide cost: GBP40,000 to GBP60,000.
Typical duration: 4 to 5 months.
Planning: may be permitted development, but this must be checked.
This can add useful space, although on a semi-detached house the appearance should be considered carefully.
Next steps
Before starting, a building owner should usually:
- speak to an architect or designer;
- obtain structural calculations;
- check whether planning permission or permitted development applies;
- check whether the loft is owned, particularly in flats;
- serve any required party wall notices in good time.
For many loft conversions, notice should be served around two to three months before work starts so that any dissent can be dealt with without delaying the project.
Conclusion
The best loft option depends on the property and the owner's priorities. Early design advice and early party wall advice can avoid delay, reduce neighbour concerns and make the project easier to deliver.