If you are sure it falls within PD then you can get on with it. Be sure it does though, and if any doubt speak to somebody that is used to and understands the rules.
Even if it is PD you still need to make a building regulations application.
bad advise....never just start a development without all the necessary paperwork in order and cover yourself. Otherwise it may well lead to legal costs and enforcement officers visiting your property, which just isn't worth the hassle.
Getting your paperwork and drawings in order is the most important aspect of any build, prior to finding a good builder or course... people tend to have the misconception that the planning and building regs is nothing more that an inconvenience and not a major factor, when its the key to the whole process.
paul