Dormer Types for London Loft Conversions: Which Is Right for Your House?
A dormer window is a structural element that projects from the existing slope of a roof, providing both headroom and natural light to a loft conversion. The type of dormer you choose has significant implications for planning permission, cost, space, and the character of your home. This guide covers every dormer type used in London loft conversions.
Velux / Roof Light Conversion (No Dormer)
Not a dormer at all — roof light loft conversions use Velux-style flush-fitting windows set within the existing roof slope, with no structural projection.
Advantages: Usually Permitted Development (no planning permission required in most cases). Lowest cost loft conversion option. Preserves the existing roof profile — preferred by conservation officers and neighbours.
Disadvantages: Limited usable floor area because of sloping ceilings on both sides. No standing headroom at the eaves. Best suited to properties with steep roof pitches.
Best for: Loft offices, studies, or single rooms where headroom at the eaves is less important. Properties in Conservation Areas where dormer projections face planning resistance.
Approximate cost premium over basic roof light: £0 (this is the baseline).
Flat Roof Dormer (Box Dormer)
The most common dormer type in London rear loft conversions — a large box-shaped structure with a flat (or very shallow-pitched) roof that replaces a section of the existing rear roof slope.
Advantages: Maximum floor area and headroom. Usually Permitted Development on the rear elevation. Relatively economical to construct. Transforms a dark, unusable loft into a full room.
Disadvantages: Flat roof requires periodic maintenance (15–25 year lifespan for modern membranes). Aesthetics — the box form is functional rather than beautiful. Some boroughs restrict the size relative to the original roof slope.
Planning: Rear flat roof dormers are usually Permitted Development subject to the standard conditions (size limits, materials, no front roof slope). Front dormers almost always require planning permission.
Approximate additional cost over roof light conversion: £8,000–£15,000 for the dormer structure.
Pitched Roof Dormer
A dormer with a pitched (gabled) roof rather than a flat one. Smaller than a box dormer but considered more sympathetic in character.
Advantages: Better aesthetics — more in keeping with period London terrace architecture. Pitched roof has longer lifespan than flat.
Disadvantages: Less floor area than a box dormer of the same width. More complex to build.
Planning: Pitched dormers are often preferred by planning authorities in Conservation Areas and areas of character, though they still require permission on front elevations.
Approximate additional cost over roof light conversion: £10,000–£18,000 for a standard pitched dormer.
L-Shaped Dormer
Combines a rear dormer with a side return dormer (where the house has an existing side return addition), creating an L-shaped roof covering that maximises floor area.
Advantages: Provides the largest possible floor plate on a typical London Victorian terrace with a side return. Often creates enough space for two bedrooms and a bathroom.
Disadvantages: More complex construction. Usually requires planning permission (because the total dormer size and the combination with the side return typically exceeds PD limits).
Planning: Often requires full planning permission. Pre-application consultation recommended.
Approximate additional cost over box dormer: £5,000–£12,000 additional.
Hip-to-Gable Dormer
Converts the sloped hip end of a hip roof to a vertical gable, then often combines with a rear dormer to create a substantial loft floor area.
Advantages: Significantly increases floor area and headroom compared to a hip roof conversion. Transforms an inaccessible hip-roofed loft into a full room.
Disadvantages: Changes the external character of the building significantly. Almost always requires planning permission. Structurally more complex.
Planning: Almost always requires full planning permission. Conservation Areas frequently refuse hip-to-gable conversions.
Approximate additional cost over standard rear dormer: £8,000–£18,000.
Mansard Conversion
Replaces the entire rear roof slope with a near-vertical (typically 70°) mansard wall topped by a shallow flat roof section. Named after the 17th-century French architect François Mansart.
Advantages: Maximum possible floor area — often equivalent to a full upper floor addition. Allows large vertical windows rather than dormer windows. Provides fully square walls throughout.
Disadvantages: Highest cost of all conversion types. Almost always requires planning permission. Changes the entire rear roofline. Structurally significant — major scaffolding and structural engineering required.
Planning: Always requires planning permission. Some Conservation Areas specifically restrict mansard conversions; others actively encourage them where they are in character with the area (many inner London Victorian terraces already have mansard roofs).
Approximate cost (full mansard conversion): £65,000–£120,000 for a standard London terrace, depending on property width.
Choosing the Right Dormer for Your Property
| Property Type | Recommended First Option | Planning Notes |
|---|---|---|
| Victorian terrace, Conservation Area | Pitched dormer or Velux | Check CA restrictions carefully |
| Victorian terrace, non-CA | Flat roof box dormer (rear) | Usually PD on rear |
| Edwardian semi-detached | Box dormer + hip-to-gable | Hip-to-gable usually needs permission |
| 1930s semi or detached | Box dormer + hip-to-gable | Often permitted |
| Flat (top floor) | Consult architect | Lease restrictions may apply |
RCB Design & Build
RCB advises on dormer type selection as part of our loft conversion design process, balancing planning viability, cost, and the space requirements of our clients. We manage all planning applications, structural engineering, and Building Control. Contact us to discuss your loft conversion.