raising floor level

[nafuk]

Hi, I need to raise the floor level over a cellar to match the concrete
floors of the rest of the house. Current floor is regular floor boards
on joists and needs to be raised approx 70mm.

I am thinking 50 mm celotex boards on top of current floor boards, then
18 mm t&g chipboard flooring, with no overlapping joists. The final
floor covering will be 19mm hard wood flooring.

Any tips or advice gratefully received.

Thank you