I've just finished building a workshop and art studio in Crewe. The council here were quite helpful.
I first put in a planning enquiry in which I specified I wanted a woodworking workshop and an art studio in two separate rooms on one level, made entirely of wood. I included a design. I was told that the dimensions of the building (27 sqm, less than 4m to the roof apex), position in the garden (more than 5m away from the house, less than 50% of garden area and more than 20m from the road) and location (PD rights in force) meant that it was within the PD rights limits and to go ahead.
However, I didn't think about BRs and after the timber wall frames were up, Building Control got in contact asking me to make sure I was complying with regs.
They were free with their advice and it turned out I was infringing the 1 metre fire rule, which is that a building of combustible materials greater than 15 sqm must be more than 1 metre from any boundary or wall. There is no discretion so I had no choice but to take the offending wall frame down and reposition it, which took a couple of months.
The good news was that as the building was less than 30 square metres it was exempt from the other building regs. The advice was, however, that the building regs were there for a reason and so I should endeavor to comply with them as far as possible even though the building was exempt. This I did.
Regardless of size you need electrics signed off.
The building was built from scratch to my own design, took 2 years and cost just under £10,000. It turned out much better than expected.
Hope this helps!