Planning a single-family house with 4 children's bedrooms

The muralist doesn't really care. What costs money are other things: third bathroom, many windows, sliding element, etc. etc. These small "pig pens" are also always expensive to build. Although one has to ask oneself, do they have to be there.

Otherwise, not everywhere is the south of the republic, you don't generally have to expect 3,000. Eleven 40 (QNG) also works for 2,500 if you are careful.

The construction partner is still irrelevant in the current situation. What you need is a good planner for your house, because with 4 children's rooms you won't find a standard floor plan.

How is the plot? Flat or on a slope?

Kind goddess. I’ll start with the second biggest handicap (the biggest being Your ignorance, which makes You an easy prey for superstition and nonsense talkers), namely the need for a four-child floor plan. With that, You are automatically excluded from adopting catalog building proposals / model houses without significant changes and will either have to expand such plans with professional support or plan individually. Name – careful, not as a link! – the development plan; I strongly suspect that You read it too superficially and hardly understand it. As I explained among other places in "How the knee wall affects the window question in the attic," the desired standing height is a poor guide for a knee wall height (and I also find it hard to believe here that the development plan invented infinite freedom in this respect).

Our "dream floor plan," which we came up with, I am attaching. The footprint would be 12.85 x 9.89 here and a gable roof on top. Knee wall at 1.80.

Forget that right away, keyword "development plan."
First, clarify your budget, what house size is even possible with it. From that, you’ll have to subtract about 40 sqm again (not for the house itself, but for the search for suitable catalog house basic models). I’ve also explained this already, in "Changing a floor plan in size." You make the choice of suitable construction method ("setting the course") during the "resting phase," and the choice of a suitable construction company only much later. Further explanations can be found here by searching the forum for my "House construction schedule," and later then "Self-award"/"Individual award" as well as "Gerddieter."
Forget hip roofs including truncated hip roofs as well, you will need the gable walls. And because of your need for living space in the attic, you won’t stay too far from the roof pitch maximum either.

The plot is about 1000 sqm and has a floor area ratio of 0.2 according to the development plan.

Go to the building authority and ask what is really tolerated and approved. With us (Berlin), also a floor area ratio of 0.2, 0.2099 was approved and 0.2100 no longer. With a good justification, it was 0.2099 +4 sqm with us.

The budget is not enough. You should limit yourselves to a smaller house.

Where is the [Bebauungsplan]?

Is there a slope?

A budget of €450,000 without additional construction costs is planned for the house.

185 sqm

.., calculatively €550,000. We can of course let the discussion circle endlessly around quality, ETICS, QNG and KfW40, (the best and most expensive that house and building fairs offer). The fact is: you hardly have the choice to avoid and speak badly of standard quality. You are currently putting the cart before the horse with phrases. Make sure you find someone who, with you, your budget and development plan, works the best out of the planning. Then you may still have possibilities to change one thing or another. But in my opinion, you won’t get around the average standard for building materials.