ᐅ Here is a floor plan for review and feedback.

Overall, I quite like the floor plan. However, a few comments are necessary, some of which I believe have already been mentioned...
The structural design concerning the living room seems questionable to me. Certainly, only lightweight partitions will be constructed on the upper floor, no masonry walls. Also, you have a bathroom directly adjacent to the bedroom wall, which I would reconsider.
There is no storage space near the kitchen; for example, where would you store beverage crates and similar items? The kitchen is clearly too small for that. It would be better to separate a small storage room and redesign the kitchen.
I don’t find the hallway too large, but that’s a matter of personal preference. However, I would enlarge the ground floor bathroom so that there is at least the option to add a shower later. If the office also serves as a guest room, where would guests shower? OK, I just read that you won’t have any overnight guests.

On the upper floor, there is a chamber marked in the master bathroom—is that intended to be a sauna? If so, I find it somewhat surprising that you are dedicating a sort of wellness area for yourselves while your children only have a modest bathroom of 4.6 square meters (50 square feet) with no storage or cupboard space. My teenage daughter feels like she has 25 shampoos and conditioners, and they all need a place to be kept...
Before proceeding further, you should obtain a more precise cost estimate.
Currently, the calculation is based on approximately 2,000 euros per square meter (about 185 dollars per square foot); I would be very surprised if the masonry garage is included in that. I think the actual price will be significantly higher than the 240 euros (about 220 dollars) indicated.

I consider 2000 € per square meter to be completely unrealistic – of course, depending on the level of fittings... However, it may be true that this building could be more expensive due to structural engineering and other factors.

marv45 schrieb:
You don’t have any storage space near the kitchen; where will, for example, beverage crates be stored? The kitchen is clearly too small for that. It would be better to separate a small storage room and design the kitchen differently.

There will be a storage room under the stairs.

marv45 schrieb:
On the upper floor, there is a small room marked in the master bathroom; is that meant to be a sauna?

Yes.

marv45 schrieb:
If so, I find it somewhat surprising that you are including a kind of wellness area while your children only have a modest bathroom of 4.6 m² (50 sq ft).

It’s not as if the master bathroom is off-limits; it’s for everyone. The children’s bathroom is mainly meant to handle the “peak times,” especially mornings before school and work.

marv45 schrieb:
At the moment, about 2,000 euros per square meter are being estimated.

Our first consultation with a local builder gave us a figure of €1300 per m² (about $121 per sq ft).

marv45 schrieb:
I think the actual price will be significantly higher than 240 euros.

We also expect it will be somewhat more expensive; we will see.

Thank you for your reply!

So 1,300 €/sqm (square meter) is completely unrealistic and, if anything, an absolute bait offer—I honestly had to laugh right away. The house shown in the floor plans/elevation will roughly cost around 300,000 - 350,000 euros.

May I ask where you are from?

Sure, from Lower Saxony or the greater Hamburg area.
But to be clear, although prices vary by region, such a difference is not realistic. Maybe if you choose the bare minimum in fittings and finishes.