Melbourne: Major/Structural Renovation Costs

Looking to get a new ppor, and was wondering what renovation costs were like in Melbourne? As far as I remember, legally, you need a registered builder to do renovations costing more than a certain amount, no?

So would that mean, registered builder for major cosmetic renos as well? Or could I just hire a project manager (and maybe register as owner-builder)? Since this is a ppor, would the rules be different? (Of course, I might be selling in a few years' time)

Also - what would the costs of a structural reno be like in Melbourne (eastern suburbs)? Things like extension (up/out), maybe moving the kitchen, etc in addition to all the cosmetic work?

I know that after a point, it becomes better to just knock-down and rebuild, but I'm trying to determine where that point is. This is in rather high-demand suburbs where older homes are common and land is worth a lot (ie seen "knock-down" lots go for 1mn+).