Concrete Split level Slab costs

1 m fill under the garage would have to be controlled and properly compacted but also you are likely to use a number of piers here.

It is not only slab itself but also footings details which will be important but a lot will also depend on the weight of your house - will it be lightweight framed house or you are planning to have bricks? Rafted slab will be more suitable for sloped split level on M class soil.

You would need to do retaining walls between your slab layers with agg drainage, gravel backfill and waterproofing - have you decided which material you will be using? Dincel probably would the be the most cost effective option here which will cost around $220-230+GST per sqm in NSW supply and install but without backfill and drainage, so roughly $2.5K will be just on walls.

You would also need to set up and connect perimeter drainage to prevent water pooling under the slab as your first step after excavation is done.

I estimate $300+/sqm all in if you want to do it right, but would be really interested to know what your builder comes back with.

It’s the drop edge beams that cost and also depends on span of the split. We have a 30m split 600mm drop.

Thanks for the feed back.
Most likely would be brick construction. Hasn't really considered light weight. I always associated that with painting.
The long side faces West so thought brick face with enhanced cavity ventilation would minimise maintenance.

I'll certainly will ask more specific questions with regards to waterproofing etc.

Lots to learn and sort through.