A good quality “dry wall” or similar proprietary basement conversion system. A work in progress but looking good and, importantly, this basement (cellar) is well ventilated with air bricks.
Always use an experienced contractor for such works. It is difficult to be totally successful with conversions of this type. Dampness problems often recur at some time. It is important to obtain a comprehensive guarantee for such a project.
This one also included a submersible water pump incorporated in the basement floor, designed to pump flood water to the exterior of the building.