L-Shaped Loft Conversions

An L-shaped conversion needs a house with a rear outrigger, a two-storey back addition with its own roof. This is typical of:

• Victorian and Edwardian terraces: The most common type, found across East Ham, Ilford and much of London.
• End-of-terrace and semi-detached period houses with an outrigger: These may also combine the L-shape with a hip-to-gable loft conversion where the main roof is hipped.

If your house has no outrigger, a single dormer loft conversion across the main rear roof is usually the right choice.

• L-shaped dormer: Two flat-roofed box dormers, one on the main roof and one on the outrigger, joined into one space. It is the most common option, and it can often be built as permitted development.
• L-shaped mansard: Both sections are built as mansards, with steep slopes of around 70 degrees and a flat top. It gives slightly more space and a more traditional appearance, which councils often prefer in conservation areas. Planning permission is usually needed. See our mansard loft conversion page for more on how mansards are built.

On a typical London terrace, an L-shaped conversion usually allows one of these layouts:

• One large bedroom with an en-suite: The bedroom sits in the main roof section and the bathroom in the outrigger section.
• Two bedrooms with a shower room: This works on wider or deeper houses.
• A bedroom and a separate study or dressing room.

The exact layout depends on the width of the house, the length of the outrigger, the head height and where the new staircase lands. At the survey we draw a simple layout so you can see what fits before you commit.

Often, yes, for an L-shaped dormer on a house in England. Both dormers count together towards the permitted development volume allowance. For a terraced house the allowance is 40 cubic metres, including any earlier roof extensions. The other main conditions are:

• Rear only: Nothing extends beyond the roof slope facing the road.
• Height: No part is higher than the highest part of the existing roof.
• Materials: The outside looks similar to the existing house.
• Side windows: Any side-facing windows are obscure-glazed and non-opening below 1.7 metres.
• No balconies or raised platforms.

Because an L-shaped dormer is large, the design has to be measured carefully to stay within the 40 cubic metre limit. You will need planning permission for an L-shaped mansard, for flats and maisonettes, for listed buildings, in many conservation areas and where an Article 4 direction applies. We calculate the volume at the design stage and can apply for a Lawful Development Certificate for written confirmation.

On a terraced house, both dormers are usually built up alongside the party walls, and the steel beams bear onto them. That means notices to your neighbours under the Party Wall etc. Act 1996. The conversion also needs building regulations approval for the following:

• the structure
• a protected escape route with interlinked smoke alarms
• the staircase
• insulation
• ventilation, including extraction in the en-suite

We arrange the inspections and hand you the completion certificate.