Hi jastlp,
We've just started reconstruction for our corner terrace. Just to share on the quote, our 'preliminaries' item is $15k, but this excludes demolition and site clearance works which cost another 7.2k. Unlike yours, our place is only 2-storey, we are doing front, back and side extension, but not additional storey.
You mentioned your RTO(clerk of works) quote is 2.5k, is piling required for your project? The quote we got is 2.8k, without piling. Mind sharing your contact with us? Thanks!