A loft conversion is building work done almost entirely inside a roof, and the roof is where we have spent over ten years. Opening one up, building a dormer into it and leaving it as watertight as it was before is a large part of the job, and it is the part a builder who is not a roofer usually hands to someone else.
The rest is what turns roof space into a room someone can sleep in. A floor strong enough to take it, a staircase with the headroom the regulations ask for, insulation, and a safe way out in a fire, which on most houses means protecting the stairs all the way down rather than relying on a window.
The shape of the roof decides what is possible. A hipped roof gives less room than a gable of the same pitch, a shallow pitch may leave too little headroom to be worth it, and the size of dormer you can build without planning permission is set by volume.