We're just about to start a major extension. The builder said that if we get 5 quotes he'd be the second or third most expensive. That said I know and trust the builder and his quality is excellent and he'll come back in a years time if something starts leaking etc and fix it. But I still have a QS doing the pricing. The builder also wanted the QS to do it in case he was making any mistakes. It's a 9-12 month project so there's a lot of things to consider.Doing an extension and renovation at the moment. Best advice I can give is get a QS to price it and then compare against quotes. Our builder is going by the QS spreadsheet but factor in 10% contingency. We got only 2 tenders back from the 5 who said they would quote. Prices varied slightly from the QS but at least you have a sanity check.