Loft Conversion Types
L-Shaped Dormer Loft Conversions
If your terrace has a narrow back section sticking out into the garden, there's a second loft above it that most people never think to use. An L-shaped dormer joins the two.
What The &Ldquo;L&Rdquo; Means
Walk down almost any Victorian or Edwardian terrace in Acton, Hanwell, West Ealing or Chiswick and look at the back of the houses. Most of them aren't a simple rectangle. There's a narrower section projecting into the garden — the rear outrigger, usually containing the bathroom and a bedroom above it.
That outrigger has its own roof, and its own loft space. It's small on its own, and on its own it isn't worth converting.
An L-shaped dormer builds a dormer across the main rear roof and a second dormer along the outrigger roof, then joins them. The result is one room in an L shape, wrapping around the back of the house — typically enough for a double bedroom at the wide end and a bathroom or a study in the leg. On a terrace, it's the most floor area you can get without going to a mansard.
Does Your House Have the Shape for It?
You're a Strong Candidate
If you have a Victorian or Edwardian terrace with a rear outrigger and the outrigger roof is pitched rather than flat. That's the standard arrangement across much of W3, W7, W13 and W4.
You May Still Be
If the outrigger has already been extended at ground level with a flat roof above — the L may work off the main roof plus whatever the outrigger still offers, though the design gets more particular.
You Aren't
If the house has no rear projection. A plain rectangular terrace gets a straight rear dormer, which is a simpler and cheaper job.
Worth Checking
Whether your neighbour has already done one. Adjoining L-shaped dormers meet at the party wall, and a conversion that mirrors an existing one is usually a shorter conversation with both the neighbour and the planning department.
What the Work Involves
Two Dormers, One Room
The main rear dormer and the outrigger dormer are framed separately and then tied together where they meet, which is the technically fiddly part and the reason this costs more than a plain dormer.
Structure
Steels spanning the main roof and the outrigger, plus the junction between them. On a terrace this almost always means work into the party walls on both sides.
The Valley
Where the two roofs meet there's a valley to weather properly. It's the detail that leaks if it's done badly, and it's worth asking any quoting company how they intend to handle it.
The Stairs
Terraces are narrow, and the staircase is usually the constraint that shapes the whole layout. Two metres of headroom is required over the whole flight and landings, and space-saver stairs are only accepted in exceptional circumstances.
Fire Safety
Protected stairway, 30-minute fire-resisting floor, fire doors and interlinked mains smoke alarms — the same as any two-storey loft conversion.
What a L-Shaped Dormer Costs
| Scope | Typical West London range |
|---|---|
| L-shaped dormer | £55,000 – £80,000 |
| With an en-suite in the leg | £60,000 – £88,000 |
These are estimates for West London, not quotes. What your loft actually costs depends on your roof, your access and what you put in the room — which is exactly what a survey is for.
An L-shaped dormer costs meaningfully more than a straight rear dormer — the second structure, the junction and the valley all add work. What you're buying is roughly a third more usable floor area and, usually, the ability to fit a bathroom without stealing from the bedroom.
Access is the other big variable. Terraces with no side return mean everything goes through the hallway, and that shows up in every quote you'll get.
Will You Need Planning Permission?
An L-shaped dormer uses more volume than a straight one, and volume is the permitted development limit that bites here: 40 cubic metres for a terrace house, measured against the original roof space. Because most L-shaped dormers are on terraces, that 40 m³ is the number to design against, and a large L can exceed it.
Permitted development also requires that no part goes above the highest point of the existing roof, that the materials are similar in appearance to the house, that the eaves are kept and the addition set back at least 0.2 metres from them, and that any side-facing window is obscure-glazed and non-opening below 1.7 metres from the floor.
And the conservation-area exception matters more here than almost anywhere, because the terraces this conversion suits are exactly the streets most likely to be designated. Roof enlargements are not permitted development on designated land, which includes conservation areas. Ealing has 30 of them, and six carry Article 4 directions that remove householder rights further. If you're in one, this conversion needs full planning permission — which doesn't mean refusal, just an application. Bedford Park in particular removes both roof enlargements and other roof alterations.
Building regulations approval is required either way. It's a separate consent from planning and it applies even when no planning permission is needed at all.
L-Shaped Dormer Conversions Across West London
The terraces that suit this conversion cluster in these areas:
L-Shaped Dormer Questions
How much bigger is it than a normal dormer?
Roughly a third more usable floor area on a typical terrace, and more importantly it's shaped so a bathroom fits in the leg without eating the bedroom.
Do both neighbours need to be notified?
On a mid-terrace, generally yes — the work touches both party walls. Notice goes out at least two months before work starts, and a neighbour who doesn't respond within 14 days is treated as having dissented.
Can it be done if the outrigger roof is flat?
Sometimes, but it changes the design and it's the first thing worth asking a surveyor. A flat outrigger roof may have been added later, and what's underneath it matters.
Find Out What a L-Shaped Dormer Would Cost
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