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> Does the house have two square meters of surface you actually can use for the purpose of sleeping, eating, living?
No. In addition, that black tar is apparently actually some sort of bitumen glue used with tiles and whoop dee doo, people used to put asbestos in it. Funny really, before I looked it up I was thinking of brushing and sanding it up which would have thrown all the fibres up in the air for me to breath in. If I breath in enough apparently it can cake my lungs and cause cancer. Fun. So now I need to get some tested.
> toilet included
what sort of toilet? a flushing one? Are you having running water in there?
> Another issue is the windows and natural light coming in
why not have the apex entirely double glazing? most glazing firms here could achieve that for you.
> 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.
yes, a back boiler, someone I know does this, he has a wood burner in his living room which heats the water in the back which goes through the radiator pipes and taps. in one way bad he has to get a fire going before he can have a bath but good he collects his own wood. However, without gas he has to use an electric oven.
> foundation
That's the easy part, dig down, dump stone, build on top.
The problem you need to be mindful of is ventilation, those turf roofs retain moisture and those beams can rot meaning you need plenty of air going through there to prevent mold, not so fun in the winter. Though, you could probably get polyethylene sheeting, like used in polytunnels, to go under the turf.