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which bit is level? the concrete? if so the floorboards may have dropped.
are any of the boards loose so you can lift them and see what's underneath?
probably worth investigating the root cause of why they are lower than the concrete section before trying to make the floor level.
bigshape-
Thanks, though the boards definitely haven't dropped. My best guess is it was a half assed extension in the 1930s. The house is built into a steep hill, they added 2 meters to the kitchen then at a later date added another utility room behind that but dug even less deeply so it's accessed by a couple of stairs that go up and are covered in the same concrete. That utility room appears to be separated by a thin layer of plasterboard and not much else from the earth behind it.
I can't quite fathom why the concrete is wonky though (steep at one side, shallow at the other), surely it wouldn't have been set at that angle. But it's definitely not level and the boards are solid/level.
Looking half seriously at hiring a breaker...
CYOA
Side profile of my floor in kitchen. The black is existing concrete. The brown is existing floorboards.
My plan is to chisel down the concrete on the red dots to get a vertical edge (because in my mind vertical edge is easier to scribe to than beveled edge?). Then lay ply (yellow) down to create a more gradual slope than the current 2 inch drop. Then once it's fairly level to cover it in a few layers of underlay (blue) and then vinyl tiles (not shown).
Is that a trash plan. What else could I do and how could I go about doing it or how could I go about cleanly chiselling down a straight-ish line into concrete?