How does this stuff happen? I feel your pain and I'm not sure what the solution is. If you have someone else do it I think it will cost you more in the long run. I would probably have the builder redo the work but write up a contract that specifically explains the mistake and his plan to rectify and stating that there will be no additional costs. I would probably hold back 20% just because he's already proven he's a nitwit and his reputation is in jeopardy. He has no bargaining power at this point and it should hurt him a little that he made such a big error.