Draw a floor plan on graph paper in a sensible scale (1 square = 0.5m) so that you can visually see your really rough ideas that you described here. One could then discuss such a proposal.
My current assessment: You are setting the wrong priorities when it comes to building a house. In planning, the heating system/ home cinema/ built-in furniture CAN be a point on the checklist, but the detailed thoughts you are having about these should really go into considerations for the room layout.
What does your daily life and work routine look like? You say that you live very withdrawn, which is your right. So why do you need a study with an attached private bathroom?
How big do your studies need to be? What professional requirements are behind that? We both work a lot at home, but without holding telephone/ video conferences and share 15 sqm. This includes archiving materials once a year to create space.
Someone already wrote here in essence that you are building a house in which you want to live, not living a life that adapts to the house.
Regarding the budget: Make an appointment at the bank and bring your preparations (income, equity, offers/ calculation examples from the internet) and move away from the “We just roughly asked this or that person.” As long as you continue like this, not only will the band stay on the bundle, it is even additionally shrink-wrapped in foil and you have no scissors to open it.
We wanted to build a solid house because our gut feeling told us so. Opinions about this are about as balanced as the distribution of Aldi Nord and Aldi Süd. If you have a reasonable architect and a confident and reliable builder, you can be happy with both types of houses.
If the drive from the Harz to a model home park is too far for you, then you probably don’t really want to build. You can usually go there on Sundays when only a few people are around. Especially without children and obligations, I cannot understand this attitude when it comes to a project in the high six-figure range. We drove 350 km just to see the brick on a reference facade because everything was closed due to corona and we didn’t want to make a decision based on 3 sample bricks. That’s what I meant at the beginning with priorities.
In summary:
- Draw the plan and ponder together over wine/ beer/ non-alcoholic drinks about what you like
- Set priorities: walls and windows before media technology
- Choose and buy a plot of land
- Make a bank consultation appointment
- Visit model houses to develop a sense of space
- Basement? I have never seen a use for it and as a child was always afraid to go in there
- Solid house/ prefabricated house? Listen to your gut feeling