When converting a loft the following things need to be taken into consideration
Roof - Typically changes to the roof to support either dormer or roof lights will require an engineers design to show how the existing roof structure will support the modifications.
Floors - In general existing ceiling joists will not support an additional floor above. This may mean that additional support will be required to ensure the new space has a solid floor and wont adversely affect the rest of the structure.
Walls - Any new floor must be fully supported by load bearing walls. Under the Party Wall Act of 1996 it is necessary to give notice to adjoining properties for some types of work on shared walls.
Stairs - The new floor must have adequate access for fire and health and safety considerations. Straight flights of stairs are the preferred type if there is enough space. Spiral staircases, Ladders and alternating tread stairs can be used when space is an issue. Steps should have a tread area of at least 22cm and rise no more that 22cm with a pitch of no more than 42 degrees. Stair width is more flexible with no major restrictions however remember that you may want to get bulky furniture up and down the stairs. Stairs should have at least 2 meters of head room.