Sell the house or keep it alone in case of a theoretical divorce?

but where exactly do I get the difference between the current selling price level and the market value?

There is no difference. The market price corresponds to the current selling price(level). That is precisely the definition of the market value, that it is the current price at the current valuation date. The determination of the market value can be done by an appraiser. And that can be influenced... (so that "their" market value tends to be higher or lower rather than being determined correctly. The market value that precisely reflects the selling price exactly does not exist. It is and remains an anticipated value.

Fact: I kept the house because the €50,000 does not have to be repaid to my father. He got his €20,000 back without interest. What he paid monthly in installments is to be seen as equivalent to rent, which he would have paid anyway, just like the renovation (which was moderate in cost). I bear the risk of the remaining financing without further claims against him. He is removed from the land register. He then gets an automatic right of first refusal on the house.

The bank is naturally excluded from the quick and amicable (only quick and timely can be amicable) settlement! Because you have a contract that you would be breaking.

The potential flashpoint lies in the mixture mentioned by of understandably present, mutually bruised emotions with financial claims. I believe that you have not yet reached the point of actually separating those, and your wife probably hasn’t either. Understandably, because you are still in the middle of the separation process with all its unpredictable developments. Therefore, based on my own experiences and with due distance, I would advise you first to carry out a real spatial separation and at the same time to aim, if possible jointly, for the sale of the house. After that, you can see what remains. All other calculations are already difficult without the emotional stress of a separation (see inheritance), but at the same time that probably won’t succeed. I see too much wishful thinking and further disappointment instead of deciding for oneself to settle matters, whether the other party cooperates or not. This can become very expensive in the end if one side reacts emotionally too hurt at some point, so I would not let too much time pass and wait until the emotions no longer influence the matter. Imagine if her new relationship suddenly faltered, how completely differently that could feel for her (not for you). Don’t be a pawn of others’ decisions, make your own, that is still my urgent advice.

I would first and foremost go to the divorce lawyer, because that is exactly what they are there for: To provide legally secure advice in the best interest of their client on a purely factual, professional level and ultimately to carry through the divorce lawsuit.

Of course, you don’t want to assume anything, but perhaps the future ex-wife is already sitting with the lawyer and drawing up a plan... while the man is still picking up non-binding ideas in an internet forum.

but where exactly do I get the difference between the current selling price level and the market value?

Don’t forget: the debts are offset. This also includes the interest-free loan from her parents: so you will also have to repay her or the parents €50,000.

Pure construction costs have so far been about €450,000. We have equity of around €200,000

With an estimated value of €650,000, a difference of €207,000 remains for you. That makes €103,500 for you. So if you get €3,500 and your equity of €100,000, it is fairly balanced.

I would first and foremost go to a divorce lawyer, because that is exactly what they are there for: To provide legally secure advice in the best interest of their client on a purely factual, professional level and in the end also to carry through the divorce lawsuit. Of course, one does not want to accuse anyone, but possibly the ex-wife-to-be is already sitting with a lawyer and planning a scheme... while the man is still gathering non-binding ideas in an internet forum.

The appointment has already taken place and everything is settled so far. There was also the advice to aim for a joint sale and, if necessary, to obtain valuations via a real estate agent/appraiser for a possible payout. In addition, to talk to the bank if we go the payout route so that I can get out of the liability. I just want to find approaches here on what else should be considered.