The only time that you'll need planning is if you're making it larger and in to a seperate dwelling. Sounds to me that you're on the verge of doing that, although not a seperate dwelling.

As for the build, you'll need an internal skin for insulation, although this can be done in timber stud form, which means that you'll lose at least 200mm overall, maybe a little more, although you'll be able to hide all services. Roof, may need changing, although i doubt that you'll be able to change it to an up and over, tiled or slated. DPC can be injected, Electrics will have to be separate to the house with a consumer unit inside the conversion. Waste, if you are cutting into the sewage line, then your water provider, or welsh water, will want to inspect before and after you've connected a line from toilet, shower etc. None of this work is that difficult.