I get this this loads of times. "Do I need plans for my garage conversion?". And the answer is invariably "No ....... but .... you need a builder who knows what to do". And its a very big "but".
It's a very simple answer to give, but thinking about it, all builders know what to do don't they? Surely no builder is going to tell a client that he does not know how to convert a garage.
Then comes the "how do I find a good builder who knows what to do" question. And the normal reply is recommendations, references, contracts, monitor the work, watch payments, employ a supervising agent etc.
It occurred to me that if the people I have done building jobs for over the years, had followed the advice I give in my professional role, then I probably would not have got any of the jobs!
People are very trusting when it comes to building work. I have got, and still get jobs on the basis of a mobile number (passed from a friend of the potential customer), a quick conversation, and a verbal estimate. Nothing else. No checks, no contracts. Nothing.
So the OP does have my sympathy. He has not done anything different to what thousands of people do. He was just unlucky.