We did it last year as part of a larger renovation - ended up being pretty expensive (more than originally envisaged) and probably the most stressful part of our build as had quite a bit of water come in as we dug down. We had to have a delta membrane to make it waterproof plus a sump pump which needs on going maintenance, There are also potentially party wall considerations depending how far you need to dig down. All that said, I love it and it is extremely useful addition to the house so totally worth it but not something I would take on lightly.
We used Mac Building Solutions and would certainly recommend -
http://www.macbuildingsolutions.com/