Thanks for the replies, sorry I was in a rush before so missed some details-

Yes, he is NOT a registered builder that is why he wants to do it that way, he just does kitchen/ bathroom renos mainly but has done some bigger jobs. his son did the plumbing in our bathroom and is a liscensed plumber, otherwise not sure about the other tradies.

We have the drawings already done and are in the process of getting permits which is why we have started getting cold feet as the legal side of the applications has made us nervous. DH is going to ring the surveyer tomorrow to get his take on it.

I just spoke to my uncle who is a plumber and worked on building sites all his life, he thinks it's ok and quite common but is checking with his brother who is a builder.

We are mostly going to pay cash as DH got a redundancy payout but it will probably be around $10-20k more than we have so will need to borrow.

I think with our non experience we need to seriously think about taking our drawings to a builder and get a quote, we are still waiting for the original guy to work out his quote, but it seems like for peace of mind, paying a bit more might be worth it. DH and I are both a bit useless with going through things with a fine tooth comb to be honest.