New building with approximately 350 sqm of living space

7 doors from the garage entrance to the living room.

Laugh, I had to count them first too.

5 storage rooms (6 if you count the corner in front of the guest WC. What is that even for?)

One for every horse

That's how kids build with Lego -

Nooo..... that's a Sims build... you can tell by the bay window and the size. Lego can't be that big because the pieces aren't there. Oh man, you really have no idea

What is the max. budget for the construction? With the house size + garage extension, additional construction costs and outdoor facilities, you are in the range of 1 million.

About the floor plan, I’ll say just one thing: This mega project belongs in the hands of a good architect!!!

Leave it. It’s pointless...

- 4 sanitary rooms for 4 people

Tss doesn't say that - when the whole family has stomach flu, you're glad if everyone can use the toilet at the same time...

I'll rather say nothing about the rest

Let's start from the beginning.
1. Why do you need 350 sqm of living space with 2 children; a total of 4 people? Wouldn't 200-220 sqm be enough?
2. I thought our floor plan was so lalala, but you never stop learning
3. €500,000 for 350 sqm = €1,428 / sqm including additional construction costs, furniture, floors, paint, outdoor facilities, garage, etc. Fits
4. Get yourself an architect and plan a nice house. If you want to build that big, then build professionally.

Just because all the desired rooms are drawn in the desired (?) size does not by any means result in a coherent or let alone good floor plan. In other words: For you, 1 + 1 is at most 0.5, but it should come out to 2.