L-shaped dormer loft conversions for Birmingham terraces

Why terraces suit the L shape

Birmingham's late Victorian and Edwardian terraces were built with a main house at the front and a two-storey outrigger at the back holding the kitchen and a bedroom or bathroom above. The outrigger has its own pitched roof, lower than the main one. A standard rear dormer only uses the main roof; adding a second dormer over the outrigger and joining them uses both.

The result is usually a main bedroom across the width of the house at the back, with an en-suite or a second small room over the outrigger. On streets in Kings Heath, Stirchley, Selly Oak and Moseley this is often the difference between one extra bedroom and two rooms.

Planning: the 40 cubic metre limit

Terraced houses have the smaller permitted development allowance: 40 cubic metres of new roof volume, including anything added before. An L-shaped dormer uses much more of that allowance than a single dormer, so the design is measured carefully before anyone assumes it needs no planning application. Where it goes over, or where the house sits in a conservation area, a planning application is needed.

Kings Heath has no conservation area at all, while much of Moseley sits in the Moseley and St Agnes conservation areas, both covered by Article 4 directions. Two houses a mile apart can therefore face quite different planning routes for the same design. A Lawful Development Certificate is the sensible check where permitted development applies.

Party walls on both sides

A mid-terrace house shares a wall with a neighbour on each side, and an L-shaped dormer usually involves both: steel beams or new joists bear on the party walls, and the outrigger dormer often runs right up to the boundary. The Party Wall etc. Act 1996 means notices go to both neighbours before work starts. Planning for this early keeps the start date realistic.

What the build involves

The two dormers are framed together so the floor is continuous, with a structural engineer's design for the steels at the junction. The staircase normally rises over the existing one in the middle of the house. Inside, the outrigger part is narrow, so it suits a shower room, a dressing area or a small study better than a bedroom.

• Measured survey of the main roof, the outrigger roof and the neighbours' walls
• Volume calculation against the 40 cubic metre allowance
• Planning permission or a Lawful Development Certificate, Building Regulations and party wall notices to both sides
• Scaffold front and rear, roof opened at the back, two dormers framed and joined, then the inside