So I don’t think it’s right that you want to pry out all kinds of personal data from the thread starter here. Instead, you could say that you have concerns about the costs and whether buffers etc. are included, or politely ask if the person is willing to share more information, but not in such a demanding tone. I don’t think turning this into a Peter Zwegat episode is a good idea.
Let’s assume the person can do the math and accept the statement that in the end 800 remain out of 4000€.
Maybe that’s why everyone is asking so unbelievably suspiciously here, but expenses of 3200€ are already very high! Maybe you have, for example due to your professions, two households, or you live in luxury, or you have included all kinds of optional costs that one could do without from a survival perspective (vacation fund, hobbies, eating out fancy every week, refurnishing your wardrobe every month, etc.). If you have a second apartment and this is unavoidable, one might question the point of having a house. Have you maybe already included a buffer in the "costs"? Or do you already have children who are also included?
Considering your income, a savings rate of 800€ is extremely low. In my opinion, if your savings rate is to be solid for home financing, it should rather be 2000€. Of that, at least 1000€ for the monthly annuity towards the loans. That leaves at least a savings rate of 500€ for unforeseen events (car needed, repairs, children and loss of earnings, additional purchases for the construction).
Of course, you can also agree on installments of 500€ with the bank and pay for it until old age, but personally I also want to have the place paid off someday.
Of course, it should be considered that eventually the previous rent will no longer apply. In return, fees, property tax, higher utility costs, and building insurance etc. will be added.
Your concern that your partner could lose his salary is justified. If that happens, the worst case is that the house cannot be kept. Hence probably the previous question about the location of the house, i.e. the implicit recommendation to build in such and such a place where you will experience at least value retention, but preferably an increase in the value of the property.
Furthermore, I can only warmly recommend choosing a very reputable financing partner. For me, these are the ones with local branches and not those with just the best conditions on the internet right now. There are also brokers, but I’m not familiar with them; I’m rather classical in this regard. In any case, inform yourself beforehand. In any case, worst cases should be addressed in the financing contract. That means that installments can also be suspended or reduced over a longer period of time! (often only for a few months).
All in all, it sounds rather unsolid on your part. If you want a more positive answer than this, you would actually have to rethink your cost calculation or possibly even reveal it.
Otherwise, I would advise the following (my personal) rule of thumb: Divide your income by two. Subtract your mandatory! costs and look for potential savings in your expenses. Consider children and desire to have children (see Google for monthly costs of children (experience values)). In the end, at least 500€ should remain for an annuity to the bank.
If only just not: bad feeling, better leave it If clearly not: leave it If yes: don’t worry