If your house has a two-storey rear addition, an L-shaped dormer is almost always the way to get the most from your roof.
Two flat-roofed dormers join at right angles, one over the main rear roof and one over the outrigger, and together they add two rooms in a single build: typically a master bedroom plus a full bathroom or second bedroom.
We design and build L-shaped dormers across London from our Fulham base, with the whole job handled in-house from first survey to final snagging.
01
An L-shaped dormer is two flat-roofed dormers joined at right angles, one built over the main rear roof and one over the rear addition, known as the outrigger. Seen from the garden they wrap the back of the house in an L, and inside they deliver the largest floor-area gain of any loft conversion type.
02
Around 10–12 weeks on site, with the full process taking roughly 18–24 weeks from your first call to handover.
03
Often permitted development on houses, provided the combined volume of both dormers stays within your allowance (Lawful Development Certificate and Building Control sign-off still required). Full planning is always needed for flats and conservation areas.
04
A site manager you know by name. Weekly photo updates by WhatsApp. The same trade teams we've worked with for over seven years. No rotating subcontractors, no agency labour walking in and out.
Each project is labelled with the time on site, the final price and the council that covered it.
L-shaped dormer adding a master bedroom and full family bathroom to a Victorian mid-terrace with a two-storey outrigger.
Master bedroom over the main roof with a second bedroom over the outrigger on a Victorian terrace.
L-shaped dormer with a double bedroom and bathroom over the back addition on an Edwardian terrace.
Master suite with full bathroom and dressing area, built over the main roof and outrigger of a Victorian terrace.
White-rendered L-shaped dormer creating a guest bedroom and shower room on a Victorian terrace.
L-shaped dormer adding two bedrooms to a Warner-style Victorian terrace with a rear back addition.
Not sure whether your back addition counts as an outrigger? Daniel will tell you straight at the free survey. book one here →
| Type | Build time | Floor area added | Planning | Best for |
|---|---|---|---|---|
| Velux | 6–8 weeks | Smallest | Usually permitted development | Homes with enough existing headroom and a leaner budget |
| Dormer | 8–10 weeks | Large | Usually permitted development | Victorian and Edwardian terraces, 1930s semis |
| L-Shaped Dormer | 10–12 weeks | Largest (two rooms) | Often permitted development | Victorian terraces with a rear back addition |
| Hip-to-Gable | 10–12 weeks | Large | Usually permitted development | Semi-detached and end-of-terrace with hipped roofs |
| Mansard | 12–16 weeks | Largest single room | Planning permission required | Conservation areas, period properties needing maximum space |
From your first call through to the day you move back in: a clear, predictable timeline that protects your home and your schedule.
Step 01
Daniel measures the main roof and the outrigger, finds the soil stack for the new bathroom, and tells you on the visit whether an L-shape works on your house.
Step 02
The design shows both dormers and the new second-floor layout, priced as a fully itemised fixed-price proposal. A 15% deposit secures your build slot.
Step 03
Lawful Development Certificate, structural calculations for both roofs, Building Control and party wall notices, all handled in-house while your build slot is booked in.
Step 04
The two roofs open in stages, each weatherproofed as we go. Both dormers complete by week 7 and finishes wrap by week 12. Weekly photo updates by WhatsApp.
Step 05
We walk both new rooms together, Building Control certifies completion, and 5% stays in retention until the snagging is signed off.
An L-shaped dormer loft conversion in London typically costs £62,000 to £80,000, VAT included. At the lower end you get a master bedroom plus a compact second bedroom with standard finishes.
Mid-range swaps the second room for a full bathroom with shower. The upper end adds a bath and walk-in shower, engineered wood floors and fitted joinery.
Because the build delivers two rooms from one project, it is usually the best value per square metre of any conversion we offer, and every quote is itemised line by line with no provisional sums.
The L is formed by two flat-roofed dormers joined at right angles: one across the main rear roof and one running along the rear addition, the two-storey projection at the back that Victorian builders called the outrigger.
You do need an outrigger, because the second dormer is built over it. If your house has a plain rectangular roof with no back addition, a standard rear dormer is the right conversion instead.
Often not. On houses, an L-shaped dormer can fall under permitted development as long as the combined volume of both dormers stays within your allowance, which is 40 cubic metres on a terrace and 50 on a semi.
Because the L-shape uses more volume than a single dormer, we check this carefully at the survey. Where it fits, we apply for a Lawful Development Certificate, which typically takes 6 to 8 weeks.
Flats, conservation areas and designs over the volume limit need full planning, and we handle that application for you too.
Around 10 to 12 weeks on site, roughly two weeks longer than a standard dormer because there are two structures and a junction between them.
We open the roofs in stages and weatherproof each section as we go, so your house stays dry throughout. Including design, certificates and party wall work before the build starts, the full journey from first call to handover is roughly 18 to 24 weeks.
Typically 24 to 32 square metres, the largest gain of any loft conversion type.
Crucially it arrives as two rooms rather than one big one: a master bedroom over the main roof, and a full bathroom or second bedroom over the outrigger. For a growing family it can take a three-bedroom terrace to a genuine four-bed, two-bath house.
It comes down to your roof and what you need. A standard dormer adds around 18 to 26 square metres as one room, costs £52,000 to £68,000 and takes 8 to 10 weeks.
The L-shape adds 24 to 32 square metres as two rooms, costs £62,000 to £80,000 and takes 10 to 12 weeks.
If you have an outrigger and want a bathroom or second bedroom upstairs, the extra spend on the L-shape buys the most space per pound. If you only need one room, or your house has no back addition, the standard dormer is the smarter choice.
Adds the most usable headroom and floor space. The most popular choice for Victorian and Edwardian terraces across South London.
About Dormer conversionsA full new storey behind a steep rear wall. The highest spec, the biggest space, often required in conservation areas.
About Mansard conversionsSquares off a sloping side roof to create the volume needed for a proper room, perfect for semi-detached and end-of-terrace homes.
About Hip-to-Gable conversionsThe simplest, fastest, lowest-cost option. It works inside the existing roof with skylight windows, often without planning permission.
About Velux / Rooflight conversionsThe L-shaped dormer depends on one feature: the two-storey rear addition.
Those outrigger terraces line street after street in Clapham, Balham and Tooting, run right through Fulham and Hammersmith, and stretch across to Walthamstow and beyond, which is why we build L-shapes all over London.
Tap an area to see what an L-shape could do on a house like yours.
Send us your postcode and a few details about your house, including whether it has a rear addition.
Daniel will book a free home survey to confirm the L-shape works on your roof, and if it does, a fully itemised fixed-price proposal follows within 7 days of the visit.
If a different conversion suits your house better, he'll say so and point you to it.
Prefer to talk? Call Daniel on 07414 648 263.
