Shell basement/foundation has stood for 10 years

Visually, it does not look completely lost. Everything has been said to the surveyor; I would have the possibilities of development according to current building regulations checked very carefully. Not that in the end, only due to changed statutes, etc., a demolition becomes necessary. That would be fatal.

Regarding the price, the surveyor can determine an approximate price for how much a basement would cost for your building project. From that, you generously deduct surveyor costs and renovation costs according to the report, and then you have the real value that the basement currently has for you.

I would offer the current owner the land price plus this real value minus a generously calculated safety margin.

Have they already expressed a price expectation?

Thank you for the answers.
That’s already good to hear.
The price has been stated. It is still €200k because of the large plot. It used to be 3 building plots that had to be combined because everything is outside the building window etc....
If I take the prices of our new development area, which is worse located, I come to a value estimate for what is on it of about €35k. For that, I cannot build on the slope. The basement already has 112 sqm of usable space including a garage. And the supporting pillars for the second half, which still needs to be poured as a base plate, are already in place. The sewer connection is also available.
The plaster will surely have to be removed extensively, but overall I consider that fair.

I have already been to the building authority.
I have the letters from the building authority that led to the building stop back then.

At the building authority, the statement was that there is an approved application that has long expired. Everything is available in copy.
If I want to build on that, I have to ensure that what was approved in the second application corresponds to the current state.
But as a layperson, it is difficult for me to assess that now. Let’s see what our architect says tomorrow... although she said we don’t need an expert report because of the condition. I am more cautious about that. What should the assignment for the expert report be called? Because I don’t even know in which direction an expert/surveyor can examine and what is necessary here. For example, a survey could make sense?
The owner’s architect brought up the issue on her own and said that the condition would be recorded and secured to ensure that the water ingress is not permanent and only comes from above.

It is certainly not the easiest plot and actually a bit too large for us, but otherwise the location is perfect and the (expensive) problems with the slope are already solved since the foundation is done.
It is a lot of money, but you do get something – provided you can build on it.
Regards

I would have assessed:

- the structure for general building damage
- compliance with the old approved plans
- conformity with the currently applicable building regulations (since you want to submit a building application again based on this) and possibly mandatory exceptions
- necessary renovations
- waterproofing from the outside

Binding surveying including precise heights is, in my opinion, absolutely necessary as part of the inventory.

Is the building today also outside the building window? If yes, I would submit a preliminary building inquiry with approximately your planned volume. When the result is available, you will have a bit more security regarding the framework within which you can operate.

Domski, thanks again! That's exactly what I needed as an overview. Very good! Now I just need an expert who has time - seems to be total chaos at the moment!

My list is without a gun... uh, no guarantee of completeness. That’s what came to mind while watching. Otherwise, I might also like the property location-wise.

The building authority stated that there is an approved application that has long expired. Everything is available in copy. If I want to build on that, I have to ensure that what was approved in the second application corresponds to the current condition.

But this does not only concern the legality of the basement you intend to build upon. I advise you to: 1. research the zoning plan at that time and 2. any possible changes to it in the meantime; 3. check in the approval which exemptions were already required there and 4. also show the plans of the basement and (even if you no longer want to implement them that way now!) of the house planned above the basement back then. Even if it was supposed to be a "city villa" or a "one-and-a-half-story" back then, these are useful hints for the fellow contributors here to develop the current building design with you. Show us the basement preferably including the depictions of where which cables, pipes, etc. go in and out. Even if "everything is supposed to be different"—for example, if an oil heating system was planned back then—these are still useful reference points for today’s brainstorming.