You don’t always need planning permission for a loft conversion but you do have to meet building regulations. These will cover such things as fire escapes, insulation, ventilation etc.
1. the loft conversion changes the outside appearance of your property
2. the new room(s) will constitute a change of use, for instance an office
3. the volume of the extended roof exceeds 40 cubic metres for a terraced property and 50 cubic metres for a semi detached or detached house
4. the loft is converted into more than two habitable rooms
5. you live in a conservation area
My advice would to be to speak with an architect who has experience with loft conversions as a good plan can make a big difference! Plans take a lot of time to draw, see image below of this loft/extension.