Floor plan design single-family house (city villa 140 sqm) on a slope with double garage

Aren't those actually two full floors?

Mono-pitched roof with 5° - if that is possible too,

That works: in the wrong direction. To avoid a full storey, you have to go down on the eaves side, which becomes more difficult with a shallower pitch.

I also don't know if it would be nice to endure 2 full rooms continuously under 2.20m ceiling height.

Certainly not: with little pitch, the same height feels like a "low ceiling height," which you would consider a "generous knee wall" with more pitch.

Aren't those actually two full storeys?

Strikingly true.

According to §2 (4) Lower Saxony Building Code, the attic is a full storey if it has a clear height of 2.20 m over more than two thirds of the floor area of the storey below.

Aren't those actually two full storeys?

This is how I calculated:

9.85 x 9.6 = 94.56 floor area.
2/3 of that = about 63 m²

63 m² / 9.85 = 6.4 m -> means 6.4 m may be higher than 2.20 m (9.6 m - 6.4 m = 3.2 m must be below if the slope descends to the north).

At 5° inclination with 2.20 m at 3.20 m building depth, I have on side 1 a wall height of 1.955 m and opposite 2.78 m. Wall thickness assumed to be 40 cm.
Regarding the wall thickness, I am unsure whether it counts as under 2.20 m – I simply assumed yes.

The 2.20 m line is drawn in and with some approximation you can see well that it really lies at about 2/3 of the area. Please refute – I am no expert. But I would like numbers rather than just "noticeably correct."
[I]

In our conversation, the house with 140sqm without a basement is roughly calculated as follows:

[*]Solid house - fully brick-faced = 255k [*]Completely retaining the plot with L-stones approx. 100m, installation, material = 20k [*]Paving work = 10k [*]Masonry double garage including foundation etc. = 40k

We will receive the exact and detailed offer shortly. What do you think of the "rough" calculation? The scope of construction services should be very similar to that of the other construction company as already mentioned, since they come from the same area and know each other's scope of construction services. Advantage?

What do you think of the "rough" calculation?

Sounds ok. Kitchen + upgrading +50k. Total at 375k - I could imagine that, if you don’t overdo it.

In our conversation, the house with 140sqm without a basement is roughly estimated as follows:

[*]Solid house - fully brick-clad = 255k
[*]Plot fully secured with L-blocks approx. 100m, installation, material = 20k
[*]Paving work = 10k
[*]Masonry double garage incl. foundation etc. = 40k

We will receive the exact and detailed offer shortly.

What do you think of the "rough" estimate? The scope of construction services should be very similar to that of the other contractor, as they come from the same area and know each other's scopes of work. Advantage?

Depends on the scope of construction services! Just because the general contractor tells you that his scope of services is similar doesn’t mean that it actually is. The price for the double garage is realistic.

How high are the L-blocks? How many rows? Will the terrain be modeled or do you have to fill and compact with a wheelbarrow?

Let’s wait for the scope of construction services!