To be fair, Some builders will point out the risks aswell as the gains
In reality, you are taking a Brick / Block cube with no insulation ( often it will have power ) and adding insulation and Plasterboard, as long as its done in a safe way, it doesnt need to be over complicated
When i was looking to move 2 years ago, the amount of " buildings and conversions " that were listed on the details, a " roof room for storage " ( it was a 5th bedroom , but not signed off ) , " garden rooms / posh sheds / man caves / home bars " ( all that were unoffical buildings without BC ) converting buildings without the " red tape " is not a new thing ( i was doing it in the early 90's and people i worked with since the 80's )
Yes of course PP and BC is great, but at times it makes the project a no-go due to the extra expense, and sometimes the risk isnt worth the money , for example I once did a double garage conversion for someone who was planning for the mother to move into it, it had a wetroom ( with toilet ) and " kitchen pod ", all finished to a high spec, they went all out, The mother was living in it for 3 weeks then passed away
the guy then turned it into a Games room and it was hardly used