I do see costs on forum raning from 2000-4000/sqm and on other forums upwards of 6000/sqm
Doing the work as an owner builder with subcontracting most works

Would $3500/m2 would be reasonable for an extension of single floor family room extension...appx 30 sqm.
Rough scope- remove brick wall ay back, concrete footing/slb , framework, 3 Degree pitche…