Dormer Loft Conversion: Cost, Planning and Design Guide UK
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Dormer Loft Conversions: The Complete UK Guide
A dormer loft conversion is the most popular type in the UK. By adding a flat-roofed box to the existing roof slope, a dormer creates maximum headroom and usable floor space — transforming a cramped loft into a full bedroom.
What Is a Dormer?
A dormer adds a box-shaped structure to the existing roof slope, creating vertical walls and flat ceilings where the sloped roof would otherwise limit headroom.
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Types include:
- Rear dormer: Most common, projects from the rear roof slope, not visible from street
- Side dormer: On the side of a property
- L-shaped dormer: Common in Victorian terraces — dormer on main roof and outrigger
Costs UK 2024
- Small rear dormer (single bedroom): £35,000 – £50,000
- Medium rear dormer (bedroom + ensuite): £45,000 – £65,000
- Large rear dormer (multiple rooms): £55,000 – £80,000
- L-shaped dormer: £50,000 – £75,000
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Planning Permission
Most rear dormer conversions qualify for permitted development and do not need planning permission, provided:
- Volume limit not exceeded (40m³ terraced, 50m³ semi/detached)
- Does not extend beyond highest point of existing roof
- Materials similar to existing building
- Not in conservation area or listed building
Side dormers visible from the road and front dormers almost always require planning permission.
L-Shaped Dormers for Victorian Terraces
Many Victorian terraced houses in London have a back addition (outrigger). An L-shaped dormer adds a dormer to both the main roof and the outrigger, creating a much larger loft space. This is the most popular conversion type in South London — Wandsworth, Battersea, Balham, and Clapham.
Frequently Asked Questions
How much does a dormer loft conversion cost?
Dormer loft conversion costs range from £35,000 to £80,000. In London, a typical rear dormer with bedroom and ensuite costs £50,000–£65,000 including all professional fees and construction.
What is the difference between a dormer and a Velux loft conversion?
A Velux conversion adds roof windows without changing the roof shape. A dormer adds a box-shaped extension creating more headroom and floor space. Dormers cost more but create a much more usable loft room.
Does a dormer loft conversion add value?
Yes. A dormer with bedroom and ensuite typically adds 15–20% to a London property’s value. On a £550,000 home, that’s £82,500–£110,000 in added value.
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