I need to lay a wooden floor in the living room of my flat, probably
bamboo or oak boards. The current floor needs to be levelled first,
due to some subsidence occurred some 30 years ago (the house was
underpinned and no further movement has been detected). The problem is
that the floor at the middle of the flat is about 3cm (1 1.4") lower
than it is at the walls.

I'm looking for the quickest solution and height of the floor is not a
problem. Is it possible to simply lay plywood on top of battens placed
over the existing floorboards to make the floor level prior to laying
the underlay and final bamboo/oak boards?

I thought I'd use 15mm plywood and battens spaced at every 40cm. Would
that work? And could I use cheaper chipboard instead?

Thanks for any help!