Your main decisions
Rooflight conversions retain more of the roof shape, while dormers and other roof alterations change the available space and appearance. Suitability depends on the existing building.
The staircase uses space on the floor below as well as in the loft. Plan the landing, door positions and escape route together.
Insulation and finishes reduce usable dimensions. Review the finished room layout, not just the empty roof-space measurements.