Plan the house first and only then buy the land?

Exactly: first read the development plan, then calculate, sketch something, reserve, leave a buffer, have the architect look at it and roughly plan a bit, then buy if he gives the green light.

Good equity capital! Excellent!
But with your expectations, that is relative again.
Outdoor facilities and double garage, driveway, possibly outbuildings ... that already costs 100,000.
And: question many things, whether it is just a "want-to-have-because-read-somewhere-or-others-have-it-list," or whether you get benefits from, for example, 4 floors.
The money can be gone very quickly without feeling any personal added value.

I see your wishes more in the outdoor area.
And your wish for over-wallpapered metal corners is not interesting at all for now.

And whether you want solar is a technical or energy-saving regulation question that should be reconsidered.

The wishlist, combined with the equity and the rather low credit limit, does confuse me. Therefore, I'll take a risk: Let the inheritance settle first and stay grounded. Your wishlist sounds like a summary of all the buildings you have ever entered.

Hi,

some good tips have already been given, and we also asked ourselves 1 1/2 years ago in which order it is best to proceed. However, a suitable plot came our way after that, on which an architect then implemented our wishes into a house.

You already have many loose wishes and ideas that an architect, whether commissioned for performance phases 1-3, can first turn into something tangible. If you then look for the plot afterwards, you always run the risk of searching forever or facing restrictions in the development, but even those can be solved with expert help.
I also think that a size under 1200sqm makes little sense with this large house.
The budget will indeed quickly approach the high six-figure sum if all wishes are implemented. However, you first have to specify those concretely.

We are at 175sqm living space, 908sqm plot, no golden faucets in middle Schleswig-Holstein (small town 70km before Hamburg) at about 550,000€ and that was reached quickly.

Best regards

First of all, thank you very much for the many contributions. You will hear from me again in the forum [Grundrissplanung]. But it may still take 1-2 years.

Half of the equity is saved up myself and the credit limit is so low because I simply don't feel like taking on more debt. We would also get a loan of 350k-400k. But I don't want that because I want to be debt-free as quickly as possible and not pay it off until just before retirement. I hate debt and I always buy cars with cash. Even if in some cases it would be better to finance the car and invest the money instead, that doesn't give me peace of mind. I'm different in that regard. But so far, I've managed well in life with this attitude.