Thank you for your answers.
Yes, that's correct, with 2 children and part-time work I also see it critically. But one must not forget that we are still at the beginning of our professional experience. Before we consider having children, something should happen in that regard. Especially since we would have to move anyway and the rents in our area for 100-120 sqm would also be around 1500€ warm.
However, my wife also says that she wants to return to full-time work as soon as possible - as of now. She works in shifts and often finishes at 2 pm and also sometimes has weekdays off. Grandma and Grandpa would also be nearby, they are already eager to help. But her plans might change hormonally later on.
This is basically the crux of the matter we are currently facing: we want to expand. So rent or mortgage, so to speak.
My wife would go from 24 km to 59 km simply, but she could also "easily" change her employer. However, she would definitely have to cover the distance by car.
For me, it would be from 10 km to 37 km, but I would take the train for most of the way.
Currently, my wife takes the train and needs between 60 and 90 minutes. I drive or bike and need 17-30 minutes by car. For my wife, the travel time would hardly change because traveling by car is quite smooth.
It's all not so easy. Maybe we should just sell the plot and buy something used nearby.
I will try to get a mostly concrete overview of the construction costs as well as ancillary costs in the next few weeks.