Finance new land next to an existing single-family house

You should use the simple past tense, because with a three-week-old child you are only just beginning to understand what a child actually costs. Personally, I would not expect the saving rate to remain unchanged given the previous lifestyle.

Apart from that: 265 K land + 450 K house + 100 K development/ancillary costs/other amount to 815 K for me, which need to be financed in total. If you can sell your current house with perhaps a 200 K profit - no idea if that is realistic - there would still be 615 K left for a bank loan. That is almost double your current financing. I see little chance of that without significant other changes. Just my two cents ...

Thank you for your assessment, which I share. That is why we are considering whether to buy the property now and then evaluate the location over the next 4 years, see for how much we can sell our house, and if necessary sell the property again. I just have no idea what such financing could look like. Current remaining debt + 265,000 EUR for the plot amount to 535,000 EUR. That should actually be feasible with my salary, right? In three years we will then know how much the house is worth, how much my wife earns again, and what the child costs us per month.

We are simply of the opinion that if we ever get the chance for such a property again, it will probably be more expensive rather than cheaper, and therefore we have to decide now for or against the realization of our dream. By buying, we postpone this decision further into the future, because financially something can still change until then.