Some of the things that are hard to assess on your own should be questions that are easily answered by reputable builders like CD Hood Ltd and their architects.  When planning your loft they will let you know about its structural strength.  If the loft has only been used for storing light items, then there is probably a need to build a new floor. The existing ceiling joists might not capable of bearing the load of an additional floor. Changes in the roof structure have to be considered and the load bearing capacity of existing walls, beams and posts re-computed too.

It is important to take into consideration Fire Code regulations. The floor, walls and doors of the loft conversion should be at least 30 minutes fire-resistant according to UK Building Code. Staircases should be enclosed of 30-minute fire-resistant walls too. The bathroom does not need to be fire resistant but if the loft has a dormer window, the exterior should be able to resist the spread of fire for at least 30 minutes. As staircase design will vary due to space constraints, the UK Fire Code specify that a staircase should be near or opens directly to an exit point and not to a room.
 
Ventilation is another important thing to consider even if natural ventilation is feasible in your loft conversion, mechanical conversion is still required under the roof tiles to prevent condensation. Ridge vents should be placed strategically to ventilate the roof space above the ceiling.
Planning well means you know your loft conversion dreams will work.