Costs & Guides
Types of Loft Conversion: Which One Suits Your House?
There are five. Which one you need is decided by the shape of your roof, not by your budget — so start with the house.
Start with the House, Not the Budget
People usually arrive at this question from the wrong end: how much can I spend, and what does that buy? It's a reasonable instinct and it produces the wrong answer, because the conversion your house can have is set by the roof before money enters the conversation.
A mid-terrace cannot have a hip to gable — there's no hip. A loft with 1.8 metres under the ridge cannot have a rooflight conversion, whatever it costs. Work out which type your house is pointing at first, and the budget question becomes answerable.
You'll often see “the four types” quoted. That's dormer, hip to gable, mansard and rooflight. The L-shaped dormer is really a variant of the dormer, but it's different enough in shape, cost and the houses it suits that it's worth counting separately — so this guide covers five.
| Type | Suits | Space gained | Typical cost | Planning likely? |
|---|---|---|---|---|
| Velux / rooflight | Lofts that already have head height | Least — no shape change | £22,000 – £32,000 | Usually not |
| Dormer | Almost any pitched roof | Full standing height at the rear | £42,000 – £62,000 | Often not |
| L-shaped dormer | Terraces with a rear outrigger | Large — wraps around the back | £55,000 – £80,000 | Sometimes |
| Hip to gable | Semi-detached and end-terrace with a hipped roof | Width, not height | £48,000 – £70,000 | Often not |
| Mansard | Tall period terraces wanting maximum space | Most — effectively a storey | £65,000 – £105,000 | Nearly always |
Velux &Amp; Rooflight Conversions
What it is: windows fitted flush into the existing roof slope. The roof shape doesn't change at all; everything else — floor, stairs, insulation, fire protection — happens inside.
Suits: lofts that already have the height. Typically the larger inter-war and Edwardian houses, and bungalows with generous roof space.
The catch, and it's absolute: it adds no headroom. If you can't stand up in your loft now, this conversion won't change that.
Dormer Conversions
What it is: a structure built out from the rear roof slope with vertical walls and a flat roof, replacing sloping ceiling with full standing height.
Suits: almost any house with a pitched roof — terrace, semi or detached. It's the most common loft conversion in the country and in West London.
Rear or full-width: a rear dormer sits within the slope with roof either side; a full-width dormer runs the whole width and gives more usable floor, at the cost of being more visually prominent. Front dormers generally mean an application, because permitted development doesn't allow a roof extension beyond the plane of the slope fronting a highway.
L-Shaped Dormer Conversions
What it is: two dormers — one across the main rear roof, one along the rear outrigger roof — joined into a single L-shaped room.
Suits: Victorian and Edwardian terraces with a rear projection into the garden. That's an enormous proportion of Acton, Hanwell, West Ealing, Chiswick and the older parts of Twickenham and Teddington.
Why bother: roughly a third more usable floor than a straight rear dormer, shaped so a bathroom fits in the leg without eating into the bedroom. On a terrace it's the most space available short of a mansard. Watch the volume allowance — on a terrace that's 40 cubic metres.
Hip to Gable Conversions
What it is: the sloping “hipped” side of the roof is rebuilt as a flat vertical gable wall, and the roof extended over it.
Suits: semi-detached and end-terrace houses built roughly 1920–1950 — the inter-war semi that fills Greenford, Perivale, Northolt, Southall, Harrow, Ruislip, Hayes and Wembley.
What it gains: width. What it doesn't gain is height, which is why it's usually paired with a rear dormer. That combination is the single most-quoted loft conversion in outer West London. Not available on a mid-terrace, which has no hip to fill.
Mansard Conversions
What it is: the rear roof is removed and rebuilt at close to vertical, with a flat or near-flat roof over. It reads as an extra storey rather than a converted loft.
Suits: tall, narrow period terraces where one room isn't enough — Hammersmith, Shepherd's Bush, parts of Chiswick and Teddington.
What it costs you: money, time and a planning application. A mansard rarely qualifies as permitted development. And the street matters — mansards are approved far more readily where they already exist.
Which Type Does Your House Point At?
Work down this list and stop at the first one that describes you.
- You live in a flat or maisonette. A loft conversion generally isn't available — see the next section.
- Your roof is built from modern W-shaped trusses. Nothing standard applies; you need a structural opinion before anything else.
- You can already stand up comfortably under the ridge. Start with a rooflight conversion. It's the cheapest by a distance.
- You're in a semi or end-terrace with a roof that slopes in at the side. Hip to gable, most likely with a rear dormer.
- You're in a terrace with a narrower section projecting into the garden. L-shaped dormer.
- You're in a terrace and you need two rooms, not one. Mansard — and check whether your street already has them.
- None of the above, and the problem is headroom. Rear dormer.
Still unsure? That's completely normal — it's the first thing a surveyor establishes on site.
Which Lofts Cannot Be Converted?
Not every loft can become a room, and it's better to know now.
Not enough head height. There is no legal minimum loft height — but there is a building regulation requiring at least 2 metres of headroom over the whole staircase and landings, with 1.9 metres sometimes accepted over part of the stair. Working back from that gives the industry's rule of thumb of roughly 2.2–2.4 metres from the top of the ceiling joists to the underside of the ridge. It's a rule of thumb, not a rule. Under about 2.2 metres and your options narrow sharply.
Modern trussed rafters. W-shaped trusses fill the loft with structural timber that can't simply be removed. Converting one means replacing the roof structure — possible, but a different project at a different price.
Flats and maisonettes. Permitted development rights do not apply to a flat, or to a building containing flats. In most cases the roof space isn't yours to convert either.
No workable stair route. Occasionally the loft is fine and there's nowhere to land a compliant staircase. Space-saver stairs are only accepted in exceptional circumstances, so this can be genuinely fatal on a very narrow house.
Very shallow roof pitch. A low pitch leaves little usable volume under the ridge regardless of the floor area below.
None of these are reasons to be sold something anyway. If a company tells you a 1.9-metre loft is a straightforward rooflight conversion, get a second opinion.
Type Questions
What are the four types of loft conversion?
Dormer, hip to gable, mansard and rooflight. Most people count the L-shaped dormer as a fifth, because it's different enough in shape, cost and the houses it suits.
What is the cheapest type of loft conversion?
A rooflight or Velux conversion, because the roof structure doesn't change and there is no external building work — but it only works on a loft that already has the head height.
Not Sure Which Type Your House Needs?
Five questions, two minutes, no obligation.