How do you start? When do you assign whom with what?

Another reasonable approach is: You develop an idea of how you would like to live (not starting with asking with what or in what). You purchase a piece of land in a location where you feel comfortable, where the infrastructure suits you, next to good neighbors, and above all, that meets the demands of the "how." Then you take your "how" to an architect and have them create a "with what" based on a budget guideline. You iteratively adjust this "with what" together with your architect. Now you have a plan, and you obtain (or have your architect obtain) the necessary permits, tender the plan or the trades, and get started.

With your wishes including [Grundstück], you could hit the million. With €300,000 equity and a good income, though, it could work. With that budget, I would traditionally go to an architect and have everything commissioned.

With your wishes regarding the property, you could break the million mark. With 300,000 equity and a good income, however, it could work. With that budget, I would traditionally go to the architect and have everything contracted out.

As I read his post, he estimates his budget at 300,000, of which about 30% is available as equity ....

I have already read a lot about it and could answer some detailed questions, but I never find anything about the first steps.

This is called a causal relationship: there is no overlap between detailed questions and first steps. Clarify the essentials first. And eventually – when the details come up – prioritize the details before researching them content-wise.

How do I know if my idea of a house fits the plot,

The building regulations of the state and the development plan or the buildings in the neighborhood tell you that. In the area between about 500 and 800 sqm of land you are only in the size range where you can even play wishful thinking; below that – roughly from 350 sqm – you are allowed to have a little wishes. And smaller than that nobody cares what you would want, only semi-detached houses are possible.

I'll make it more concrete:

How do I know approximately what my dream house will cost without immediately paying several thousand to an architect?

You talk to the architect and give a budget guideline. Either he is ambitious to solve it for you or he will immediately decline. Such a preliminary talk costs nothing. Choose one who is excited about your project. He gets excited when he is allowed to design. Give a very clear and at the same time detail-wise generous framework. So know how you want to live.

How do I know if my idea of a house fits the plot, or respectively how many sqm of living space would be right for me?

The size results from the “how.” For example, two of us manage perfectly well with 115sqm. Say goodbye to the idea of "price per square meter." Rather build your small dream house with a generous price per sqm instead of a price-per-square-meter-optimized "villa," where you don’t end up living as you wish. Some hobbies naturally need sqm, like a snooker room, for example.

Nana Hampshire. 115 for two is not tight. And never forget, having is better than needing. And beauty fades, the hectare remains. That also applies to sqm. For mahogany fitted wardrobes, the money will be enough in 25 years, it won't disappear. But building an additional missing room is more difficult.