How do you raise the floor on a concrete slab?

answer

You would lay timber floor joists acroos the the size would depend upon how high you need to raise the floor. Then you'd lay floor boards over the joists. the joists would be layed every 400mm and would require noggings to reduce movevement and 'creaking'. The floor boards should be of a tongue and groove type for the same reasons. The cheapest floor bords are made of chipboard and are 8' by 2' and they should be glued.

If you wanted to raise it a matter of a few inches then you could just screed over the top of it. If you wanted just a few mm then a self leveling liquid can be applied.