How to increase part of existing internal concrete slab by 300mm
Without thinking too deeply about it and just basing it off of similar circumstances I would fill with about 8" of compacted crush, lay 2" insulation, lay radient pipes and pour 2" of gypsum concrete. It's not as strong as standard concrete so the compactness of the substrate is critical. It's thermal characteristics is far superior. It's not suitable for basement slabs or slab on grade applications because they require a structural component. Another great feature of gypcrete is that is pretty much self leveling.