I think that this is the way to go because it's really hard to catch all of the things that can go on on site if you're not a professional. I built (ok got fellas in to build) an extension many years ago and I didn't catch that the drainage was inadequate until after they had completed and left - so I had to get that done separately and it wasn't cheap. Every builder I've talked to since has given me the impression that they would have spotted this upfront, and I think I believe them.