Here everyone says "Du" and this is your thread where you can chat however you like.
The stair climbing issue has been discussed often. You are building a huge house. When you are old enough that you can no longer climb the stairs, you either build new or add on or move straight into a nursing home. Also, there are stairlifts - they work great. My grandma is now 99. Until three years ago she lived in her house and climbed a steep staircase every evening. Then she partitioned off a corner downstairs in the living room and slept there for a year. Now she is in a home. The idea of living cramped for 60 years because of that one year is nonsense.
Furthermore, the question immediately arises what you want to do with the rooms upstairs otherwise? Should ghosts live there? Please don’t say "rent out," because that is just the next nonsense, since you need a separable staircase for something like that and you are far from that and it requires even more space.
What is the WC in the garage supposed to be? Is your architect related to the plumber?
I would strongly cut back here, otherwise this will be a bottomless pit. You already notice now that your budget is not enough. A house gains quality of life when you plan fewer rooms on the same area, but those perfectly. Think about what you really need and what not.
It is common, for example, to plan a shower in the guest WC and then the children or the dad use the bathroom downstairs. Whoever doesn’t want that and has the money plans a kids’ bathroom and a master bathroom, but only a WC with a sink downstairs etc. You have to plan according to your budget and consider how to manage with less, because currently you are already over the limit and it will be much more expensive than you think.
What exactly is included in the 380K budget? Is that just for the house or also for paths, terraces, garage, utilities, in other words, ancillary building costs? What about the costs for the private road that mentioned, are those known?