Does the house have two square meters of surface you actually can use for the purpose of sleeping, eating, living?>>34440
So I would create a loft above the crossbeams. I would divide the downstairs to bathroom (toilet included) - hall - kitchen, upstairs would be a bedroom and a living room/office. Maybe some additional storage space could be gained in the upmost triangle.
The chief problem is making it not feeling cramped. Maybe a 4m by 7m inside space for the downstairs would be enough. Judging by the proportions on the photos, it still would be bigger than the original design.
Maybe I would go further and add the hall "outside" the entrance, or enlarging it in that direction, but that would need a modification of the structure to a T shape. That would add a couple of square meters.
With crampedness height also a factor. I dunno, maybe 220 cm should be a minimum height for the ceiling downstairs. We have a building with about 2m, and feels bearable, but I don's spend much time in there.
Another issue is the windows and natural light coming in (also could add to the cramped feeling if it's not enough).
Third problem is the floor. Gravel is too... outdoorsy for the indoors in case a structure which is built for living and not working. It's only pro that it massages the soles nicely when walking on it barefoot.
Fourth: plumbing and electricity, but that wouldn't be much more different than a "normal" house.
Next one is the heating. Maybe a hearth-like furnace downstairs at the wall opposite the door and a couple of radiators (one in the bathroom, another upstairs - the hot water pipe would go through the kitchen) would be enough.
Not exactly the last one (there's some minor, for example one cannot nail a nail into the stone walls), but probably it's more important than the chief problem: foundation...
Oh, actually the most important: plans usually have to be handed in for an inspection, and get a permit. Not sure they would allow me to build this. I don't know what are the bureaucratic regulations telling about the process.