How to make unlevel slab floors level..
Out GC Alon’s advice on making an unlevel slab floor level:
First investigate this, as this is quite an extreme situation.
However, you may be able to address this (losing headroom). Remove all flooring material and apply lightweight concrete.
Lightweight concrete is practically liquid when applied (i.e. it is self-leveling) and can be installed as thin as 3/4″.
Link to discussion on Houzz: