What should I do with my house and land

Where do you get the money to finance the construction of the apartment building or a part of it? The amount you have to borrow will be significantly higher than 100k. Can't you refinance the 100k again on the house/land?

I get quite a bit for the shares of the land, as 3/4. My old loan still runs for 3 years, then I can refinance and possibly sell the house even without residential value.

I get quite a bit for the shares of the property, as 3/4. My old loan still runs for 3 years, then I can refinance and possibly sell the house even without residential value

Once again the question about the timeline. When is all this supposed to take place so that your ex gets his money? Who participates in a private developer model for a condominium? When will you have the three partners together? Nothing will happen until there are four of you. Who plans the house – all four of you or do you dictate what the other three have to build? There is no developer behind this with all the legal safeguards. If only one partner has a problem and cannot continue building, who does it then? Do you take over the apartment? Do the other three parties then wait 1.5 years with a shell until a fourth comes? And so on. You are still extremely naive here. That’s not bad, but the more answers you get here, the more your ideas are likely to waver...

Not final yet, but I know the approximate amount. We have been arguing about it for 5 years and it is now also in court

But you don’t even know yet whether you have to pay at all?
If so, I would choose option C, divide the property and sell half with the option that a building may not exceed a certain height (if the sales proceeds are sufficient).

When my mother is no longer alive and my daughter moves out, the house is anyway much too big for me and with the large garden, it also means too much work for me.

But it’s not that far yet, and who knows what else will happen in life.
Why don’t you simply stay living in the house or the vacant apartment and thus remain close to your mother and have plenty of time for her as long as she is still here. In this situation, such a house project, possibly even a multi-family house, does not sound like stress relief; plus the divorce issue.
If the court wanted to oblige you to make a payment, it would not evict you and your daughter from the apartment.
If you really pursued your previous solutions, I see you sinking even deeper into the whirlpool; to me, it seems a bit like a reactive action prompted by the imminent verdict.
Five years of marital dispute and now you are looking for the next drama with a multi-family house or other contortions. I am quite sure that there are much more prudent solutions for this, even if they don’t always bring you the best financial outcome.

If the house has always been yours, then why [Zugewinn]?

I say, yes, hard to calculate. If she had more debts before, that can already count as an increase in value. And the appreciation. Inheritances have to be assessed differently.

Tip for goodwill: if it is reasonably clear what has to be paid and lawyers differ only by a few euros proportionally, then settle out of court. That ultimately saves significant costs. Then regulate it contractually. And then you can arrange EVERYTHING. Including deferral of payment, etc. Or take out a [TA loan] for 3 years (if the bank agrees) and refinance completely in 3 years.