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You need to clear of the debris under what was the wall and hearth to assess.
Could be that you can reduce the height of the hard standing and batten onto it at the same height as the joists holding up the wood floor. This will then support your new floorboards.
Whatever happens you will have a height difference between the kitchen and the floor that will need some kind of reducer bar.
That sounds like it would involve a lot of digging and a lot of building - and I assumed the wall was built on top of some kind of foundations. Could the floorboards not be laid on top of that somehow?