Converting a loft could be one of the most cost-effective and straightforward ways of adding extra living space into your house. Converted loft can be used as a master bedroom, playroom or a home office. There are couple of details to keep in mind when planning to convert a loft. 

You will need to know whether your loft is suitable for the conversion or not. To do so, inspect the roof space in order to check roof's structure and dimensions of available room around you. The conversion would depend on the head height, the roof pitch, the structure of the roof, and any other little details such as water tank or chimney stacks.
In order to check how much available head height is available, measure the space where there is headroom of 2m or more available between the bottom of the ridge timbers and the top of the ceiling joists. If there less 2m of head space you might need to consider raising the roof or lower the ceiling in the rooms below. Raising the roof would require a planning permission. 
Establishing the structure of your roof before converting is very advisable. In pre-1960s homes the typical structure used for the roof construction was the traditional framed type, here the rafters, ceiling joists and supporting timbers were all cut to size on site and assembled. This type of structure would be the most suitable for a conversion as you could open it up in an easy way and strengthen the rafters and add supports after.
In post-1960 house prefabricated trussed rafters were quite usual. If your roof structure is made of trussed rafters then opening up the space will require added structural input, such as steel beam, between the load-bearing walls. 
It is important to remember that all loft conversion require Building Regulations approval in order to ensure that safety measures are in place, as well as complying with Part L, ensuring the U value targets for thermal efficiency are met. Even though there is no minimum ceiling height in place to meet Building Regulations, the recommended practical ceiling height would be 1.9m. The new staircase leading to the loft conversion will have to comply with Building Regulations to ensure the appropriate head height has been allowed. Under Park K of the Building Regulations, the edges of the loft stairs could be reduced to head height of 1.8m to allow for sloping roofs, with the centre of the flight of stairs being at a height of 1.9m. 
If you are considering to convert your loft, I say you would need to get some help. Get in touch with an architect or designer, who provide plans, structural engineer to specify and oversee some of the more technical elements and of course the building team.