Glossary
Dormer windows and the rules for London loft conversions
A vertical extension on a sloped roof, creating headroom and a window. Most common loft conversion type.
A vertical extension on a sloped roof, creating headroom and a window. Most common loft conversion type.
A dormer is a structure projecting from a sloping roof with its own vertical window, creating full head height inside the loft. A rear dormer is the most common loft extension in London, permitted development on many houses within a 40 m³ volume allowance for a terrace or 50 m³ for a semi or detached house.
Class B of the GPDO permits a roof extension of up to 40 m³ on a terraced house and 50 m³ on a semi-detached or detached house, measured as added volume against the original roof. The dormer must not extend beyond the plane of the existing roof slope on the principal elevation fronting a highway, must not exceed the highest part of the existing roof, and must be set back at least 200 mm from the original eaves.
Materials must be similar in appearance to the existing house, side-facing windows must be obscure-glazed and non-opening below 1.7 m, and the rights do not apply in conservation areas, National Landscapes or on flats and maisonettes.
A full-width rear box dormer gives the most volume and is the London default on Victorian and Edwardian terraces. An L-shaped dormer follows the rear back addition as well as the main roof and typically yields a second room. A flat-roof dormer set within the slope, or a smaller pitched-roof dormer, is the usual compromise in a conservation area where Class B is withdrawn.
A protected stair enclosure with 30-minute fire resistance and fire doors to habitable rooms off it, an escape window where the storey height allows it, structural design for the new floor and dormer, Part L insulation in the new roof and cheeks, and Part K stair geometry — 42-degree maximum pitch, 220 mm minimum going, 2 m headroom.
A worked Hounslow example of dormer: with 30 conservation areas — the Chiswick Conservation Area and the Bedford Park Conservation Area among the best known — London Borough of Hounslow (Hounslow Council) sees dormer constantly, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Where dormer pushes a Hounslow scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks.
| Volume allowance, terrace | 40 m³ |
|---|---|
| Volume allowance, semi or detached | 50 m³ |
| Set-back from eaves | 200 mm minimum |
| Side window glazing | Obscure, non-opening below 1.7 m |
| Conservation area | Class B rights withdrawn |
| Our loft package from |
Not usually outside a conservation area, provided the volume and set-back limits are met.
Yes. The allowance is cumulative against the original roof.
Not under permitted development where the roof slope fronts a highway.
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