L-shaped dormer drawings in London

Where the roofs join

A loft conversion normally has to keep the original eaves and step back from them. For an L-shaped dormer the rule is different where the roofs meet: Eaves-retention and 0.2 m set-back do not apply where the enlargement joins the original roof to the roof of a rear or side extension.

That condition is why the L can run unbroken across the junction instead of stopping short. It also means the design can look good from the garden, with a single clean face rather than a series of steps.

The detail at that junction is the hard part to build. Valleys, flashings and the structural support all meet there, so the sheet that shows it gets our most careful checking.

Volume and height limits

Permitted development limits the roof you can add: 40 m³ (terrace house) / 50 m³ (any other house) additional roof volume. A terrace house is the lower figure, and an L-shaped dormer on a terrace can reach it quickly.

Height is also capped: No part higher than the highest part of the existing roof. The L has to sit below the ridge, which constrains how much headroom you gain.

If the volume is too large, you have two choices: shrink the L, or apply for planning permission. A planning application through planning drawings is common for big terrace lofts, and we will show you both options with their trade-offs.

Conservation areas

As with every dormer, the national rule is plain: Class B roof enlargements are not permitted development on article 2(3) land. In a conservation area, an L-shaped dormer is therefore a planning application.

Officers read it against the street. They look at how the dormer sits on the outrigger, how much roof remains visible from neighbouring gardens, and whether the materials suit the terrace. Detail matters more than the headline size.

We check the address, the conservation area and the planning history of your road first, then tell you which approach is open. The cost is a fixed quote after a free constraints check, VAT included.

Party walls, stairs and fire safety

Terraces share their roof structure, so party wall notices are the norm. The rule for cutting into a shared structure is: Party structure notice at least 2 months before work. We prepare the notices and drawings with party wall notices so the programme is protected.

The stair needs headroom: Loft conversions where 2.0 m cannot be achieved: 1.9 m at the centre of the stair, 1.8 m at the side (half width). With two rooms in the roof, the stair often rises through the landing of the outrigger, so we test its position on the model.

A bigger loft means a bigger fire plan: New storey above 4.5 m by loft conversion: protect the full escape route with fire-resisting doors (min E 20) and partitions (min REI 30). That affects the doors on the landing and the specification of the walls, and it goes through our building regulations drawings.

Structure and cost

An L-shaped dormer carries more load than a plain box and spans an awkward corner, so the engineer's design is central. Steel sizes come through structural calculations, and we coordinate them with the drawings.

We do not publish a standard price for this type, because the work depends on the roof and the junction. You get a fixed quote after a free constraints check, VAT included, and we redraw at no charge until approval if the council asks for changes.

A second MCIAT chartered technologist checks every sheet before it goes in. Drawings are normally ready about a week after the survey.

L-shaped dormer drawings by council

Every borough has its own view on roof extensions over outriggers. Find your council under architectural drawings by area for the local conservation area and Article 4 figures.

Not every terrace needs the full L. A plain dormer loft conversion is often enough, and loft conversion drawings compares the four shapes.