Newly built single-family house with 150 sqm living area, Saarland

I also don't find the door under the stairs as access to the bathroom successful - in old houses, that is the position of the door to the cellar stairs. That would be more suitable as access to the [Technikraum].

: Thanks for the welcome.

OK, until 2 hours ago I still assumed that a 9.5x10.5 meter house is spacious.

But not without a basement and with 15 rooms/areas. 150 m² for 4 people is also rather standard. If it even were a real 150 m².

... and you better not waste too much space for a sense of spaciousness. Without children, it is also hard to imagine how much storage space you need.

What disadvantages are you talking about? OK, until 2 hours ago I still assumed that a 9.5x10.5 meter house is spacious...

I'll try again with the note about the detailed questionnaire requested here, although I don't really like forms.
Your mentioned house size means nothing at first as long as you don't know what is supposed to happen inside. Maybe the rooms mentioned by are not children's rooms at all, but reading rooms, fitness rooms, or terrariums for the iguanas.
Are 5 people really supposed to live in there, i.e. 2 adults + 3 children?
Shouldn't one first detach from these terms "gallery" or "villa" and thus predetermined things and look at what MY family needs in order to live individually and appropriately and how that fits into MY budget?
Therefore again: Just show what, how, why... then you'll find great people here and it will be a nice house for YOU! The more details you give, the more quality you get back.

The gallery/air space is a wish of mine that I would like to have.

I have wanted a 50 x 10 meter outdoor pool for years as well; but unfortunately that also has to wait; instead I am currently building a nice house for us.
Someone probably put a bug in your ear and you hope that a hole in the ceiling inevitably creates a nice gallery or makes your house more beautiful. But so far you overlook that children are supposed to live in very small rooms and the common room is literally crying out for space.

The house is designed for currently 2 people, maximum 4

Then why the current room layout on the upper floor?

As already written, it will be about 0.5 meters bigger in every direction.

Oops... then a different floor plan? You can do it like that but simply making it bigger does not necessarily and to the desired extent improve the result.

OK. Thanks for the constructive criticism, I will reconsider the gallery, even though it's only about 5 sqm that are lost.

The three rooms on the right side will then become two rooms of about 15 sqm each. The idea was already like that anyway.

Which disadvantages are you talking about? OK, until 2 hours ago I still assumed that a 9.5x10.5 meter house is spacious..

9.5x10.5 is also big enough for 4 people. But just not like that.

A straight staircase is really something nice and IMHO very pleasing to look at. We are also currently in the "pre-planning phase/idea finding" for a house of a similar size and have found that there are simply better options for this size of house.

If you are only planning for 2 children, then first turn the three rooms into two. Then they also have a reasonable size.

You could also put the gallery in this case where the storage room is on the upper floor.. above the entrance area.

Without children, it is also hard to imagine how much storage space you need.

Right, because a child needs 3x more storage space than two adults :D, at least it feels like that.