Floor plan of a single-family house with a separate apartment on the south slope above the street

Exactly for this reason, we are planning with a granny flat to help us with the financing

[*]Living space is in high demand in our region

As a rule, it is a naive calculation. Operating costs still have to be deducted from the rent, and the income must be taxed. Smaller and without a granny flat is usually cheaper.

Apart from that, I do not build new to immediately get roommates into the house. Why? Spectators in the garden, possibly rental stress, mutual consideration necessary, etc.

As already mentioned, the budget seems a bit low to me. The basement and the earthworks and the massive retaining walls that significantly drive up the price are already included.

A roof terrace at the property boundary is somewhat questionable. Does the staircase help with that?

The roof terrace, I believe, may not extend up to the boundary.

Do you get to the living area through the Essen lock?

That is called a private location. It is very undisturbed.

Office door directly at the stairs?

If occasional noises don’t bother you, it doesn’t matter.

WC within sight of the kitchen?

Probably only a problem for sensitive people.

You “almost stumble” into the kitchen when you enter?

I think that’s great if the kitchen is a central living space.

I don’t like the very long access through the basement because it would be hard for us to keep it tidy and all people would be led through the "storage area."

The granny flat would have to bring in a lot of rent to be worthwhile – as a financial aid instrument, it is questionable. For own use and renting out the large part as a second pension, it can eventually be worth its weight in gold – depending on how it goes. Basically, there is nothing wrong with granny flats. We always assume that it is rented to a potentially disturbing "stranger," but that is only one use for a granny flat.

I think the roof terrace is not allowed to go all the way to the boundary

Don’t call it a terrace – that is a lawn area with fall protection over an underground parking garage.

The roof terrace is, I think, not allowed to extend up to the boundary

That's right!

Edge development: according to Art 6 BayBO

Unless the article states otherwise.
What does the mentioned article say?

Regarding the pictures:
The house reminds me of "Vacation on the Farm".

But if it fits the surroundings and is desired, I think that's okay and appropriate. It wouldn't be my style either, but that doesn't matter.

Which 2m line on the upper floor do you mean?

The standing height on the upper floor

Which staircase do you mean?

[*]From the basement to the ground floor?

Both! They narrow on the left and right, on the ground floor you practically enter a hallway only 2 meters wide. That would work for two people, but not for family life, I think. At least a surprise cake can’t even be baked.
The same applies to the staircase entrance on the ground floor: passing the chill area, you have absolutely no privacy within the family (except in the bedroom).

We are building in the expensive southern part

Well, then I just had the déjà vu of a plan from this week in Lower Saxony.
That won’t work. Elaborate earthworks and paving, roof terrace… granny flat, that lets the budget run dry before you move in.

Regarding the parking spaces: I don’t see four!
Could you please submit a drawing with the house on the plot including the street and a north arrow? I don’t get the orientation, I somehow can’t assign the plot on the plan (green area and what is around it? Where is the street?)
Are the foreign cars supposed to park in front of your entrance? Is your entrance next to the granny flat terrace?
I wouldn’t want to rent the granny flat as a couple.
Alone as an interim solution if the rent fits, gladly. It is too small and too close to you owners to stay there. Also the thumping of the stairs next to the kitchen wall… That means a lot of comings and goings and wearing out, so renovation needs, possibly also rental gaps.
The house is also not reasonable inside, e.g. direct view into the toilet from the kitchen, although nice dining area and the upper floor will also work (except for the bathroom layout, which can be changed), but the granny flat personally displaces my own privacy too much.

North arrow is on the site plan. Street where the cars are parked. ;)