Advance payments for a possible future construction site

As I said, the plot is actually huge, but due to the hillside location with quite varying slopes, it may not be that easy to build on. If I already had the survey of the plot, I could work with different scenarios

Why do you need a survey for that? I don't understand. Take a handful of stakes, mark off "your" future section and plan a bit.

Why do you need a survey for that? I don't understand.

Because of the slope. The elevation profile is especially interesting, as the incline varies quite significantly in places.
So far, there isn't a 'my future part,' but I get a part that I like (provided I don't divide the property disadvantageously).
Then I can see where and how to place a house nicely and efficiently and choose that part accordingly.
For example, to be able to build with a basement or to have to cut away or fill in as little as possible (provided the location also personally appeals).

So far, there is no 'my future part,' but I get a part that I like (provided I don't tear up the property disadvantageously).
Then I can see where and how to place a house nicely and efficiently and choose the part accordingly.
For example, to be able to build with a residential basement or to have to cut away or fill as little as possible (provided the location also appeals personally).

Then a survey won’t help you much either. A residential basement in the slope is also not exactly cheap.

And just "seeing" where and how you can build a house depends not only on the survey or the terrain profile but is also largely defined by the development plan or the surrounding buildings. And there are people who have studied exactly this topic for many years.
You can have ideas, but whether they are feasible and "affordable" is another matter.

About the property:
- approx. 7000sqm undeveloped plot with a south-facing slope, but I would only keep a part of it (1000 - 1500sqm).
- Residential areas to the south and east, a swimming pool to the west, and basically a land consolidation path to the north with arable land beyond that
- Municipality has already been asked, they have no objection to building here (the older sister had the idea 2 years ago but then abandoned it)

Unfortunately, the overall description does not sound like a plot fully classified as inner-area property :-(

I'm less concerned about building a house single-handedly, but rather about eventually showing up at the architect with a bag full of information and saying: "These are my ideas, this is my property, these are example scenarios, these are my habits, ... and here are already all the necessary documents prepared"

However, the house could easily soon become part of divorce proceedings if the lady of the house was only involved in the "finalization" but basically everything was already planned conceptually. There are reasons why it is said before the altar "say loudly and clearly 'yes, I do!'", and not "please sign bottom right" :)

Due to my professional situation, I am currently not near my parental home/property (about 1 hour drive), but I really imagine it would be nice to return and stay in the coming years (time horizon 4-5 years). The question is whether it makes sense to take the initiative in advance and start planning a house?

It only makes sense if you are sure about returning in 4-5 years. What happens in that time? And... how old are you anyway? I mean, a lot can happen until then. If you were in your early 20s and just starting your first job or studies, then the future is completely open. If you are somewhere in your 30s, then most of the whims have left your head, but a new job or a new love can still bring you a 1 hour drive in the opposite direction. Who knows. What you can plan now, in general, is how you want to live and what a corresponding floor plan could look like. However, regarding the property, I would only take the initiative in terms of planning, meaning checking things like what you are even allowed to build there and what other steps might still be open for development. With a rough sketch, then show up on site at the city office at some point and explore feasibility. Is the property free, or does the parental home still stand on it as well? Now the property is not yet in your possession. If you have to buy it from the family, it might be advantageous to buy it now due to rising land value reference prices, which brings us back to the first paragraph I wrote.

Unfortunately, the overall description does not sound like a property whose entire area is attributed to the interior :-(

I agree. Definitely talk to the city first about what you are allowed to build where.

There are reasons why it says at the altar "say loudly and clearly 'yes, I do!'" and not "please sign below on the right" :)

Sorry, I just find that incredibly funny :D:D:D. But you’re of course right, it’s just expressed in an entertaining way.

And... how old are you actually? I mean a lot can happen until then. If you were in your early 20s and just started your first job or studies, then the future is completely open. If you’re somewhere in your 30s, then most of the nonsense is out of your head, but a new job or a new love can still bring you a 1-hour commute in the opposite direction.

I’m in the middle. I’m 28. Professionally, I just started a job where I am confident that I can work in this field until retirement (not that I have to) and also shouldn’t have major problems changing the position.

Is the plot free, or is the parents’ house still on it?

The plot is free, a large meadow, undeveloped on the outskirts of a village. It wouldn’t be a purchase, but a gift. Because it’s undeveloped, I don’t expect a high land value (building land costs about 120€/m2 here in the new development area). But the location is really nice (in my opinion). Basically, the municipality had already inquired a few years ago about buying this meadow to make a small residential area with 9 units, which my grandparents (then still owners) declined. But maybe it really makes sense as a first step to approach the municipality to ask in which area building a house would even be conceivable.