I have a house in the village, whose first floor has a considerable unevenness, more than 3% without a doubt. The question is that the house is seated directly on a stone slab, with hardly any foundations. As the height does not allow it to level the room, I would have to fill in the lower part and trim the upper part, but I do not trust that cracks may appear or that the floor may soften if I make a repointing. What would be the best solution to secure the walls and lower the floor level?
In reference to what you say, depending on the amount of unevenness of the floor, you could opt for several options;
Between 2 and 5% you should be able to remove the floor from the highest part (3/5 cm) easily and apply a cement called self-leveling. This process should not affect the walls.
Between 5 and 15% you should be able to remove the floor from the highest part (11 cm) and apply a mortar or concrete slab (as a filler) to level the surface. Once hardened and dry, apply self-leveling (as in the previous step, if you are looking for a good leveling).
For unevenness of more than 15-20% you should make some holes in the load-bearing walls (if you say they have no foundation), place iron grating and fill with concrete (every 2 m approx.) and shore up the structure for safety. Once the walls have been underpinned and secured, the ground should be chipped to level, gravel should be applied as drainage and plastic should be used for insulation. You mention that the floor is rock and these floors tend to accumulate a lot of moisture. If not insulated, moisture could rise to the floor and walls. The final step would be to apply a concrete slab with iron mesh to make a new level floor and apply the finish of your choice.