You have already understood financial responsibility quite well.
I’ll pick up a few points you mentioned that haven’t been addressed yet:
The additional costs increase due to new insurances -> let’s say a total of about €400 per month (garbage and property tax are also still there), in addition to the loan and reserves (this was recently a topic, also in a "I wish for a house" thread). Of course, you can usually adjust the loan over the years if the salary has been adjusted, but you have to say upfront: more buffers are consumed before and during the construction than you think (the buffer is usually gone before getting to the details of what the buffer is actually supposed to serve). Additionally, most banks do not finance furniture, so not even the kitchen; commitment interest as well as double payments come into play, usually at a time when no one calculates anymore because they simply don’t want to know. Then you have to buy a lawn mower, new lamps for the house wall for example, mailbox, a privacy screen, etc.: usually from ongoing salary or retained equity, and this adds up to values in the multiple four-figure range. But if the salary doesn’t generate that much, then a house is no longer the dream you imagined. Maybe then the parents will support, but those are extras you don’t want to think about now.
Now to your wishes:
As I said, we don’t need big extras when building the house
I also don’t think we can manage €350,000, so the house would have to be cheaper—only where to save?
Hehe, basement, double garage, and living area of the house... I have already noticed: the attitude towards a basement has a north-south gradient in Germany. Strange though: where building a house is already comparatively very cheap, builders are more willing to forego the basement and rather build with more light.
Your wishes are already not small and not adjusted to your budget!
To the house calculation come the small "you-only-build-once" extras, then garden, driveway and incidental construction costs.
And I will also go back to work or take on a mini-job as soon as the kids are in kindergarten, but probably a third child is planned (that’s why the 150m2), so it could still take some time.
With three kids you clearly need living space.
however, in the village/area where we want to stay, there is almost nothing to buy
If you want so much, you shouldn’t limit yourself mentally to too narrow a local structure and draw boundaries.
I don’t know anyone who mentally restricts themselves to one village (whether for work or housing). Even if the infrastructure is good, it may be that the neighboring town with the same infrastructure is much cheaper. You should also look about 20 km further.
In the end, what matters to you is the supply, which is not available. You can’t change that.
But you can change your way of thinking.
Go to the bank and get a non-binding offer on how much money you can get.
And then take to heart again what I wrote above and all the advice of my predecessors.
Best regards, Yvonne