What Is a Rear Dormer Loft Conversion?
Introduction
The rear dormer is the most common type of loft conversion in London. If you've walked down a residential street in a Victorian or Edwardian neighbourhood in the past few years, you've probably seen them — the box-shaped structure added to the rear slope of a terrace's roof, creating the flat-roofed extension that characterises most loft conversions in the capital.
This guide explains what a rear dormer is, how it's built, what planning position applies, and what to expect in terms of cost and programme.
What Is a Rear Dormer?
A rear dormer is a structural addition to the rear-facing slope of a pitched roof. It creates a vertical-sided extension of the roof that contains windows — typically Velux-style rooflights in a flat roof, or upright sash or casement windows in a vertical face.
The key characteristic of a rear dormer is that it extends the head height and floor area of the loft space significantly by raising the rear slope of the roof. Without a dormer, most loft spaces have insufficient head height for habitable use across most of the floor area. A rear dormer solves this by replacing the sloping rear roof section with a taller, flat-roofed box.
What's Included in a Rear Dormer Loft Conversion?
A complete rear dormer loft conversion typically includes:
Structural floor upgrade. The existing ceiling joists of the floor below are typically undersized for use as a habitable floor. A structural engineer designs new floor joists — usually steel beams with timber joists spanning between them — that are installed across the loft space to create a proper structural floor.
Dormer construction. The rear slope of the roof is cut back (in part or fully) and a structural timber frame is erected to form the walls, roof structure, and internal structure of the dormer. The flat roof of the dormer is typically a GRP fibreglass or EPDM rubber membrane.
Roof windows. Velux roof windows (or similar) are typically installed in the flat roof of the dormer to bring natural light into the space. Alternatively, a vertical-faced dormer has upright windows in the front face of the dormer structure.
Staircase. A fixed staircase must be installed to access the new habitable room. The staircase typically rises from a first-floor bedroom or landing and involves some reconfiguration of the floor below.
Insulation. The new loft space must be insulated to meet Building Regulations thermal requirements — both in the dormer structure and in the retained pitched roof sections either side.
First-fix MEP. Plumbing (for an en-suite bathroom if included), electrical cabling, and heating connections are installed during the first-fix stage.
Plastering, decoration, and second-fix. The loft space is plastered, decorated, and fitted out with bathroom fixtures, joinery, and lighting.
Planning Position for Rear Dormers
Many rear dormer loft conversions can be built under Permitted Development rights, without needing a full planning application — provided:
- The dormer does not exceed the height of the existing roof ridge
- The dormer does not extend forward of the principal elevation of the roof
- The materials are similar in appearance to the existing house
- The property is not in a conservation area (conservation areas require planning permission for rear dormers)
- The property is not listed
If PD rights don't apply, or if there's any uncertainty, a planning application or Lawful Development Certificate application is the right next step.
What Does a Rear Dormer Cost?
The cost of a rear dormer loft conversion varies depending on the specification, the size of the property, the finish level, and whether en-suite plumbing is included. We don't publish price ranges in our content as costs change with market conditions. Contact us for a proper estimate based on your specific project.
Ready to understand what a rear dormer would cost for your property?
Contact RCB Design & Build for a structured project discussion.
📞 07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk