attic living space

Structure
The ceiling structure of a home is not normally built the same as it would be if there were to be a living space above.  The wood structure may need to be beefed up or a new floor reframed on top of the ceiling for additional support.
Weight
That extra addition will add a lot of weight.  If dormers or gables are added, the roof weight will be increased and put pressure on the foundation.  The foundation may need more support (i.e. wider).
Height
A minimum height for habitable space is 7 feet.
Clearance
Door frames and the like need to be properly positioned to be able to be used in this space. Appropriate clearance with the pitches is a critical aspect of remodeling an attic.
Insulation
The framing usually used is 2×6 and that is not deep enough to fit standard insulation.  It can either be reframed in 2×8s, add 2×2s to the exisiting 2×6s or sister 2×8s on to the exisiting 2×6s.

photo:barbmcmahon  on Flickr