200-300 sqm prefabricated house or passive house? Manufacturer? Construction company?

In terms of price, you can get both a good prefabricated house and a solid house for that. First, think about what you want, what advantages and disadvantages each construction method has, what suits you better, and then look further. Visit prefabricated house exhibitions, get advice, and request offers. Personally, I find staggered living levels nice but impractical. A flat roof wouldn’t be my choice either, because space is lost and I just don’t like it – but it’s a matter of taste.

... At first, we thought of a generous gable roof house, but now we actually like a flat roof house with 2 or 3 staggered floors even better. Due to 2 children, a hobby room, and a study, we need a bit more space (>200 sqm). ...

Is a flat roof as a 2 or 3-story building allowed according to the development plan?

If you calculate an average room size of 15 sqm for hobby/work and kids per room, then 160 sqm should be enough. What exceeds that is breathing room and space that feels good ... but not necessary.

... A flat roof wouldn’t be my thing either now, because space is lost there and I just don’t like it – but that’s a matter of taste

What kind of space is lost there? Do you mean the floor area under the sloping roof?

Yes. Depending on the pitch of the roof and the knee wall, you can make good use of the space up there and not just let it become storage space. We use the space under the roof, for example, as a nice play/TV room for the kids. I personally find it nicer and cozier than, for example, in the basement (which we don't have).

Thank you for your answers!

We don't like a normal flat-roof house that much either, but at least offset.

What we mostly like are the Huf houses.
We'll probably take a look at them today in their model village...

P.S. I just checked -> Only 2 full floors are allowed.

So after the viewing we can say that we really like the Huf Art 5. Unfortunately, €500k won't be enough for that... Do you have any other good alternatives in mind?