How do I bathroom / kitchen / extension? etc.

Ply is normally > chipboard if he can afford it, stronger and stiffer for a given thickness. Also less prone to fall apart if it gets damp (see above).

It all depends on the exact usage case though, we're having underfloor everywhere so we're using 18mm T&G cement boards (No More Ply) because they have better thermal and acoustic properties. They're fireproof too. Just more expensive and heavy - I had to unload a half pallet of them yesterday which is half a tonne so that was fun...