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

There should be a solution then. Do the banks cooperate with that?

Of course the banks cooperate. But that means there will be a new property valuation, the unpaid amount will be subject to a non-acceptance fee, the property will be re-evaluated, and finally, maybe a part of the already paid-out money will be reclaimed. In the worst case, that could lead to your insolvency.

Conclusion: stupid idea

For me, these are also more like two semi-detached houses of different sizes. Maybe it would work if you actually made it a [WEG], then only the smaller unit can initially be built, financed, and encumbered.

Looks like two independent residential units. Just build your "granny flat" without a basement and sometime later the main house with a basement.

Can you pay for the granny flat without financing?

I think it will have to/can go in that direction. I initially discarded it because you 1) lose 40-50 sqm of basement 2) have to do a slab and dig a hole anyway 3) the effort of building a basement is higher when connecting to an existing building without a basement (undermining, settlements)

Of course banks agree. But that means there will be a new property valuation, the unpaid amount will be subject to a non-acceptance compensation, the property will be reassessed, and in the end maybe part of the already paid out money will be reclaimed. In the worst case, this could lead to your insolvency.

Conclusion: stupid idea

You are probably right that it will be more expensive. Whether the bank drives you into insolvency, I don't know, as long as they get the installments. They certainly have no interest in marketing my basement construction

For me, these are more like two differently sized semi-detached houses. Maybe it would work if you actually form a condominium, then only the smaller unit can be built, financed, and encumbered at first.

If it's a separate unit, I believe so too, it would basically be easier regarding the bank issue. Disadvantages are, as written above, no basement or other preparatory construction for the main dwelling. But it will probably come down to this.

A question to the group. Would one first approach an architect or the bank with the topic? From hearsay, I heard that banks want a finished concept and then calculate financing based on it, but do not want to be bothered with ideas and "would have, could have".

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

but not to scale.

If you improve where it essentially fails (stairs shown too small), it will become clear that this building arrangement (in these dimensions) will not succeed anyway. I generally do not consider an apartment of this size desirable to design specifically as a maisonette – especially not with regard to usability in old age. If you still want to pursue your basic idea, I would first consider a fundamental change of course regarding the building arrangement: towards a separate apartment 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 construct the upper floor as a shell or add it later (although raising the roof also costs money, a shell only plastered on the outside might therefore be more economical). In doing so, the ground floor part of the future main apartment could be used as a rented separate apartment during the single-story stage (later living area / wardrobe / guest WC or children’s area).

Conclusion: stupid idea

Note well: a specialist agrees with me on that, I would take that seriously as the original poster.