There are four types of loft conversion in common use and they form a ladder: each one adds structure, space and cost to the one below it. Which of them your house allows is decided by the roof you already have, not by preference, and the survey that settles it is measuring three things: usable head height under the ridge, the type of roof structure, and where a staircase can land without ruining the floor below.
Rooflight loft conversion, the cheapest
Sometimes called a Velux conversion after the common brand of rooflight. Nothing is added outside the roof: the floor is strengthened, insulation and a staircase go in, and windows are set into the existing slope. It changes the outside of the house least, is the least likely to need planning permission, and only works where there is already enough head height.
Dormer, the common one, including an L shaped loft conversion
A structure is built out of the slope to create vertical walls and full head height across part of the loft. Box, full width and L shaped variants suit different roofs, and a rear dormer is often permitted development on a house. It is the type that produces the largest gain for the smallest structural change, which is why most British conversions are dormers.
Hip to gable and mansard, the structural ones
A hip to gable rebuilds a sloping end into a vertical gable, adding head height along the side of the loft, and suits semi detached and end terrace houses. A mansard reconstructs the rear roof into a near vertical slope and produces a full storey; it is the most expensive and almost always needs planning permission. Both involve masonry and both keep the scaffold up longer.
A roof lift loft conversion, when the head height is not there
Where the existing ridge is too low for a usable room, the answer is to raise the roof rather than to squeeze the room. The covering comes off, the structure above wall plate level is rebuilt higher, and a new roof goes back on. It is the most expensive way to convert a loft and the one most likely to need planning permission rather than falling inside permitted development, because the ridge line and the roof volume both change. It is worth pricing where the house is otherwise right and the alternative is moving, and it is worth abandoning early where the survey says the walls below would need underpinning to carry the new load.
A garage loft conversion, and the floor that has to be built first
The space over a garage looks like free floor area and is not, because a garage ceiling was never designed as a floor. The joists are usually sized to hold a plasterboard ceiling and some boxes, so a new floor structure is the first line of the quote, and on an attached garage the way in matters as much: a staircase has to come from somewhere inside the house, and the room it takes is part of the cost. Then insulation to current standards, a fire separation from the garage below if it stays a garage, and the headroom question a low pitched garage roof usually loses. It works best where the garage is being converted at ground level at the same time, because one job pays for the stairs, the structure and the building control route.
Questions people ask about types of loft conversion
How much head height do I need?
Enough that, after a new floor structure below and insulation above, the finished room works and the staircase meets the regulations. A surveyor measures the existing ridge and works down; a rule of thumb quoted from a website is not a substitute.
Does the roof structure matter?
A great deal. A traditional cut roof has open space between rafters and purlins; a modern trussed roof is filled with webs that carry the load and cannot simply be cut out. Converting a trussed roof needs a structural design to replace what is removed, and that changes the price.
Which type adds the most value?
The one that produces a room somebody wants, with a compliant staircase, an escape route and a building control certificate. An uncertified loft room is a storage space with a ladder as far as a valuer is concerned.
Is a loft conversion value calculator any use?
For a rough sense of direction, yes. For a decision, no. What a conversion adds depends on the ceiling price of the street, whether the house gains a bedroom that an estate agent will count, and whether it loses one downstairs to the stairs. A calculator knows none of those. Use one to see whether the idea is in the right order of magnitude and then ask two local agents what the finished house would be worth.