Home financing about 500K - Can I afford it?

I wouldn't have expected that much. I would have estimated 10-15K.

And even if it had only been 10k, just for the shell construction, small surcharges on other trades add up very quickly and in the end, there will be 50-100k more on the clock. -> for the same house. Across federal states, I do consider differences of a factor of 2 or more quite possible, depending on the area / city.

So we currently need to finance 520k with 5k net income and personally we see absolutely no problem with that for us. As soon as there are children, it will also have to work for a few years with 4k net. But we also belong to the group that thinks 100€ for clothes must be enough. But: despite a lot of personal contribution, we expect total costs of at least 720k, so that would still be a crucial point for you whether a new build is really possible with that amount? In the background, we have family like you with property values in the seven-figure range. A guarantee would NEVER!!! have been an option for us. However, we do accept support in the form of labor, equity contribution, and family loans.

At the moment, we are also wondering whether we might possibly build a duplex with very good friends; the question would be whether the whole thing for a plot [Grundstück] at 300k and a duplex [Doppelhaus] at 550k plus additional costs would be feasible completely for 1 million, meaning each party 500k.

Currently we are also wondering whether we might build a duplex with very good friends, the question would be whether the whole thing for land 300k and duplex 550k plus additional costs would be feasible completely for 1 million, meaning each party 500k.

Yes, a duplex could be cheaper. In the worst case, though, it could cost the friendship.... It doesn't have to be, but such things have happened quite often.

180sqm for 230K? In which year did you build?

We built in 2017. However, the prices come from 2015/2016 (planning year). This was possible because we built with Heinz von Heiden (a volume builder of the rather affordable kind). Additionally, no excessive extras. No basement, no finished attic (cold attic). These are the major cost drivers for a house. The disadvantages of such a provider must of course be known to you.