Unless you're in a Listed Building Kath, then there should be no Planning issue. There's no "Change of Use". It's simply still Domestic use.
Building Regs definitely. Rather than the expense of Architects drawings etc, have your builder put in a "Building Notice". No drawings, but the Building Inspector will discuss it with the builder, and work it out between them.
I guess you have a polycarbonate roof? That really would need to be upgraded to a conventional one with modern insulation.
The same might apply to the single skin brickwork as well. It's discussed on site, and really at the discretion of the B/Inspector. Preliminary discussions with him would tell you.
The other thing that makes it difficult to quote remotely, is where your drains are. They may not be handy, thus adding to the cost.