Single-family house with a possible granny flat?

Ah, I actually overlooked that, shame on me.
Imho even worse

Yes, you have nicely calculated it for yourself.
150 - 170m² + 60m² granny flat will cost you significantly more than the estimated 450,000€.
For a total of 220m² I would expect around 550k.
That will be tight with your salary, or rather impossible.
Leave out the granny flat. I don’t understand why someone who already has enough bad experience with renting would plan a granny flat.

I am confident that I won’t have any problems with my parents as tenants. I know the bank does not count rental income as income but with the rental income, I think it would work out.
550k is quite steep though. I don’t know if I want to get into such heavy debt. The loan wouldn’t be a problem, my aunt has the connections (Sparkasse).

Because the parents are supposed to move in there. Even though I wouldn't wish such closeness to either my parents or parents-in-law, I still believe that the problems arising from this are not comparable to those caused by "difficult" tenants who are strangers. Given the OP's age, one can also assume that the parents will live for a few more years.

Is the granny flat rented out to the parents? --> Rental income
Do you receive rent for the father-in-law's apartment? --> Rental income

Even if you don't count these 100%, the two rents would probably bring your monthly payment into a more realistic range for the 750k€ + X investment.

That’s how I calculated as well, but I don’t want to convince myself unrealistically and then end up looking like a fool. With existing properties, it is very rare that a granny flat is attached, which is why we wanted to build.

170m² for 3 people seems quite a lot to me. You should be able to manage with 120m² or, if more children are planned, with 140m². Then it should become a bit cheaper again.

Well, 140sqm would be enough for me, I think my wife has the extra wishes with a walk-in closet, pantry, and at least 15sqm bathroom. On the ground floor, a guest toilet without a shower is enough. So we do have some potential savings. EL are also not included at the moment. For the electrics, flooring, and painting work, I would only need the materials, as there are suitable craftsmen in the family who want/will help.

Ah I actually overlooked that, shame on me.
Imho even worse

You think so? I have a very good relationship with my parents and imagine it would be very nice to live "under one roof" again. I also have to mention that I am Turkish and my wife is German. I am used to living with the whole family like that. My wife isn’t, that’s why the granny flat, otherwise a 200sqm house would be fine for me too.

Well, that is something very personal, the comment was not meant seriously. The bank definitely takes sustainable rental income into account. If the parents pay rent, it looks a bit different. But aside from that, I think your wife will have to limit her wishes somewhat. For many extravagant desires, the income simply isn't enough; building in Swabia is anything but cheap. The estimated 260€/m² floor area unfortunately also seems cheap to me or simply no longer current, but of course it varies greatly by region. Basically, you have a good income and have already saved up quite a bit of equity. You will be able to fulfill your dream of building; I think just a little more brainpower needs to be put into the financing concept. Is the father-in-law’s apartment debt-free and can it be used as collateral?