Dormer
Flat roof or pitched roof dormers can be used to increase available height in part of the attic area. This increase may then facilitate better access or usage.

Gable/mini hips
Sometimes minor changes to roof design can dramatically improve potential e.g. Raising ridge lines, forming split level section(s), changing roof pitches, forming mansard roofs etc. More radical approaches may sometimes be involved for which the benefits sometimes greatly outweigh the costs.

Roof redesign
Hip roofs are a common feature of houses here and can limit the potential for conversion. Changes to hips, or their elimination by forming a gable wall can transform the usage potential.

Others
Projects involving mezzanine floors, lowered ceilings, relocation of existing stairs (wall or partition) and/or change would fall under this category though they might also incorporate any of the roof alterations above