1) Is there enough head height?
People are often disappointed by how much space they have to actually stand up in, whilst there may seem a lot of room in the loft space to walk around, the joists that you are walking on are technically only for very light loads usually only around 75mm deep and to support the ceiling plasterboards. A second set of joists needs to be placed next to the ceiling joists to support the floor loads and they may raise the deck by at least 150mm more so if there is a no load bearing walls below.
The roof needs to be upgraded with insulation to become a habitable room, which will amount to around at least 125mm thick, some of which may be fitted between the rafters, but there needs to be an air gap of 50mm left between the insulation and the felt to prevent moisture building up.
So you may find that you lose around 250mm of head height when the loft conversion is carried out.
Don’t forget you’ll have to accommodate a staircase leading up into the loft – to make the best use of space the new staircase should rise above the old one and not from within an existing bedroom, but this is not always practical, because you need to maintain a 2.0m head height above the existing and new staircase and to achieve this, the bottom of the stairs needs to start directly above the existing stair, and this is usually inside and existing bedroom.
Also consider whether there are water tanks and plumbing in the loft, because the heating and hot water system may have to be replaced with a sealed system.