First floor plan L-shaped house (190m²) with granny flat (80m²), basement

The staircase of the granny flat somehow doesn’t work.

Yes, it should. It’s just drawn incorrectly. Several stair parts were put together by the program and on the ground floor the arrow was placed wrong.

Is there also a plan showing how the house should be placed on the plot?

Probably like this.

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And you can see: it doesn’t fit. Not even with the paper walls. Further up it was already calculated what would be possible, without considering that parking spaces, a storage room for bicycles, possibly a platform at the entrance, etc. would still have to be built.

at least 3 m)

You said it yourself.
Of course, you can sketch with paper walls, but everything should fit on the plot. Now in theory and practice you still have 45 cm exterior walls plus "real" interior walls, which are load-bearing at the staircase, at least a 20 cm wall between the dwelling units. Your stairwell is now about 160 cm wide by just under 2 meters, where a space-saving staircase would fit. I personally do not plan less than 200 x 240, only exceptionally smaller in row or storage-space houses.

Show the surroundings. With 34 cm walls that is important to know.

Yes, that would be sensible. Best a screenshot of the street and surrounding area from Maps.

On mistakes in thinking:

a lot of input from our side (it is not exactly a standard construction method)

So far, what irritates me is the crazy space consumption of the house by the 36 sqm large terrace, so that only these 3 meters remain all around.
Also, what irritates me in the granny flat, which is meant for the father, is this staircase to a basement that is declared as living space just because a light well is drawn there. The side windows will be windows with light shafts where possibly a car might (or must) still park.

Open living area with adequately sized kitchen and cooking island.

If you calculate, you have 3 tall units plus 180 cm of counter/work surface plus island. I find that rather little. We need more working and storage space in a (tidy) 2-person household.

Challenges: basement windows, especially in the area of the granny flat; preferably no view windows from the granny flat into the garden of the main house

You don’t have a view from a basement window. With a light well window you only look at a wall or into the sky. There are no lines of sight there.
There is also no garden.

Bathroom main house: We also want a nice large shower bathroom downstairs.

Something like “nice and large” also needs space, which could be possible, but not in this, in my opinion, small area on the ground floor of the main apartment.

Sufficiently large rooms available;

Is that so? Just because a value comes out by calculation doesn’t mean that those are sufficiently large rooms. The child’s room and the office of 2.40 m width by 6 meters length are, in my view, bottom level and should not be like this.

We are unsure whether the kitchen & living area is not too open (nowadays an L-shape for this area is modern)

What is modern does not matter. An L-shape is more practical because it zones or offers visual protection and with 4 people you can also “get out of each other’s way” sometimes. At least no surprise cake can be baked here. And constant presence of one resident can be annoying over the years because he is just there.

On the concept:

Granny flat should be completely separated from the main house and connect to a residential basement

Granny flat completely separated is nothing new and should be like that. Whether one can call a basement with a small light shaft in front of a window a residential basement is another matter. For me that would be a hobby room for children or a guest or office room, but not a permanent living room for family members.

Wish, no view windows to the garden of the main house

As said above: either there is no view or you solve such a problem fundamentally with planting. The granny flat doesn’t even have a terrace!

It was initially about being able to estimate if everything would basically fit into the rooms and if distances between kitchen units and cooking island would work.

I would say: no.
Again: no exterior walls considered for the plot, no parking spaces. Staircase too small, shower bathroom too narrow, poorly zoned open room. No quickly accessible storage room. Poorly cut rooms upstairs, also the bedroom and dressing room are rather narrow for the arrangement.
Basement is somehow too much (if you leave out the technical room)

(or self-takeover, if the kids take over the place ;)).

Do you want to enter the gloomy staircase into the dark basement room which is supposed to become your bedroom at age 60 in the granny flat?

Ps: it may be that I scaled the property incorrectly (typed 19 16 instead), but the main tenor of my contribution remains the same.

I am just a financier. 270sqm living space for under 800k plus additional construction costs as well as outdoor facilities and garage/carport will be difficult. Especially since you have expensive wishes.

I come up with about 115sqm on one level (living space, not floor area, because that's not feasible without walls). That makes an oversized 345sqm. Your basic tone is of course further confirmed by this. Even if a partial basement already exists, it just doesn't fit.

And if you rethink the concept, for example, moving the granny flat with a small roof terrace and a comfortable staircase that can be upgraded for old age into the upper floor, and placing the bedrooms of the main apartment in the basement with a generous light well and possibly a partial area for office and lounge on the upper floor, then you can of course question why you turn a flat plot into a hillside plot by enclosing one side completely with a light well. However, in my opinion, it makes more sense than completely sinking the granny flat into the basement, especially since an exit/terrace should be aimed for there as well.

Let's ask this: how high is the basement below the ground surface? How high would the light wells be accordingly or need to be secured in the garden? And the entrance platforms: how high would the entrances have to be so that space would have to be planned for this on the plot?

Where should one start. Many square meters, little comfort

The granny flat is dark, no garden, no terrace. Guest room with 7 sqm, where should bed and wardrobe fit in.
What has the father done wrong?

The office is a hallway
What is the lounge upstairs supposed to be for.
A lot of traffic area, lounge + corridor bigger than the children's rooms.
I find the rooms dark, especially child 1 with the 2 loopholes.
In the dining area, the chairs are next to the kitchenette when in use. Minimum depth for a dining area is table width + 1m on each side, i.e. 3 m with a 1 m wide table.
How deep are the kitchen cabinets?

Overall, it can be smaller and more efficient. I would already change the shape. Privacy is nice and all, but you separate one family member more rigorously than strangers in an apartment building or in townhouses/semi-detached houses. The price for that is extremely high.

so it’s not quite as bad with the kitchen and the dining table.

For the kitchen, a 4m wall unit is possible, a 3m island in front of it, 3m for the table and chairs, leaving 3m for the kitchen unit, row spacing, island. Many only have 9m room length for kitchen, table, and couch in a row.

And with 8.65 room width, 50m2+, that should be more than enough.

Here's my suggestion. A basement apartment is out of the question. Here, also with almost 80 m2 on the ground floor.