My advice to you is to find somebody on checkatrade if you are busy, they have lots of builders who will give a very reasonable price and are often available for an immediate start. The quality and attention to detail you would get is no more than you deserve.It’s just a general enquiry on how to find the right builder for this job. I have all the drawings and approvals. Life is busy. I didn't have time to check back on this yet. I would have expected this sort of question first up.
Good luck with your project