Putting rooms in the roof space can be a cost-effective way of maximising the internal space. It is also a way of gaining rooms where the local planners have set the height for the ridge. For example, they may dictate that the ridge is not set above those of neighbouring houses.
If you do go for a room in the loft you will need stronger roof trusses to support the weight of the floor. This will increase the cost of building the roof structure.
