Is only part of the building permit enforceable? How long is the building permit valid?

So you should at least have an offer from a general contractor, or do you already know what your project will cost?

If you improve where it essentially fails (stairs depicted too small), it will become clear that this building arrangement (in these dimensions) will not succeed anyway. I generally do not consider designing a granny flat of this size as a maisonette desirable – especially not with usability for elderly people. If you want to pursue your basic idea further, I would first consider a fundamental change regarding the building arrangement: towards a granny flat entirely on the ground floor and a main apartment on both floors. Then you could build over the entire area in construction phase I and either build the upper floor as a shell or add it later (although raising the roof also costs money, so a shell only plastered on the outside might be more economical). You could then use the ground floor part of the future main apartment as a rented granny flat in its single-story stage (later living area / wardrobe / guest WC or children's area).

and only develops 2-3 rooms to live in (which can then be called a granny flat or not).

as long as they receive the installments. They certainly don't want to market my basement construction

Not "calls", but rents out. This part can already contribute to the repayment.

Maybe just the shell and roof will work. You don’t urgently need windows and heating if you’re not living there.
Except, of course, for underfloor heating, which you could install but then leave switched off.
But I would really get advice from a general contractor on that.

So for the granny flat 150k and you have that or need to finance it?

The bank will give you exactly €0 for a shell and then see. So you have to finish building anyway and not just partially.
If you don’t do that and leave a half-ruin standing or occupy only part, the bank has very little security. If you then cannot offer additional security or partial repayment, it will simply be auctioned. No matter what has to be sold. The bank does not care at all about your fate. Nor what the house brings in.

I want to live there and then expand the rest.

I will go back to the drawing board and it will probably come down to smaller dimensions and a complete build with initially only 2-3 rooms being finished.