A dormer loft conversion builds a structure out of the roof slope so that part of the loft gains full head height and a vertical wall. It is the most common conversion in British houses because it turns an unusable triangle into a room, and it is the one where the difference between a good job and a poor one is least visible from the street: everything that matters is in the structure, the insulation and the junction with the tiles.
Box, full width and L shaped dormer extensions
A box dormer is a single structure with its own small roof, usually at the rear. A full width dormer runs the whole width of the rear slope and buys the most space, at the cost of how the roof reads from behind. An L shaped dormer wraps a rear dormer around an existing back addition, which suits Victorian and Edwardian houses with an outrigger and produces the largest gain of the three. Each is a different quantity of structure and a different planning conversation.
What holds a dormer loft extension up
The dormer's floor is the loft floor, and the loft floor of a house was not designed to carry a room. New floor joists, usually spanning between load bearing walls and often carried on steel, are put in beneath or beside the existing ceiling joists. The dormer's own walls and roof then sit on that floor. This is the part of the job a structural engineer designs and building control inspects, and it is not the part a brochure photograph shows.
Where dormer conversions go wrong
At the junctions. The flat or low pitched dormer roof has to drain and it has to be flashed properly into the tiled slope on both cheeks and along the top. Cheeks are commonly finished in tile, lead, zinc or a modern single ply, and each has a right detail and a wrong one. The other recurring fault is insulation: a dormer has thin walls and a shallow roof, and getting the required thermal performance into that thickness needs the specification to be right rather than assumed.
Inside dormer loft conversion: the parts people forget
The dormer itself is a small building: structure, a flat roof, insulation, a window, cladding to the cheeks and a weather detail where it meets the tiles. Around it sits the rest of the conversion, which is usually the larger half. New floor joists carrying a domestic floor load, a compliant staircase in a position the floor below can spare, fire separation to the escape route, insulation to the sloping ceilings, and ventilation. A quote that prices the box and not the conversion is not a conversion quote.
Party walls and neighbours
On a terrace or a semi the work usually involves cutting into or building off the party wall, which brings the Party Wall etc. Act 1996 into it. That is a notice and an agreement rather than a permission, and it runs in parallel with planning and building control. Start it early, because the notice periods are real and they are a common reason a loft conversion starts later than the owner expected.
Questions people ask about dormer loft conversion
Is a rear dormer permitted development?
Frequently, within volume limits, set back from the eaves and not extending beyond the plane of the front roof slope. Flats and maisonettes do not have those rights, and designated areas often have them removed, so it is worth confirming for your own address.
How much head height do I need to start with?
Enough under the ridge to make a room once the new floor and the insulated ceiling take their share. A survey measures the existing ridge height and works down, which is why the first visit should involve a tape rather than a price.
Can I have an ensuite in a dormer?
Usually yes, and it is common. It needs a waste route down through the house, which is easier where it can follow an existing soil stack, and that route should be agreed in the design rather than found on site.
How long does a dormer loft conversion take?
Several weeks on site for a straightforward one, longer where steel, a difficult staircase or a party wall award is involved. The design, the approvals and the party wall notices commonly take longer than the build, so start those first.
Can any loft take a dormer?
Most can take something, but the ridge height, the roof structure and the position of the staircase decide what. Modern trussed rafter roofs need more structural work than traditional cut roofs. A survey before you fall in love with a layout saves money.