To make the room livable, based on UK legislation the two main points are how would you be able to get out if a fire happened in the other part of the dwelling. UK Building regs if it’s a habitable room, such as a gym, you either need to direct access out via a suitably sized window or a door or alternative create a protected stairway within the dwelling.
And it is properly tanked to prevent moisture ingress.
Tough to resolve those points if the cellar is fully under the ground and then the added issue of ventilating the room .