Floor plan of a single-family house, slight slope location, northwest orientation

128 sqm are also mentioned on the same page. I wonder where this number comes from.

Yes, I have seen it. I assume that 128 comes from smaller sand-lime bricks. The house apparently comes from a semi-detached house series, the 154-house is a custom house, possibly calculated based on the 160/180 sand-lime bricks. Or they mean 228 including the basement, which I can't imagine because basements are always calculated separately.
You can make a built-in wardrobe, but it’s not worth it in a room where you can barely stand at the front.

The property has an elevation difference of 3m over 35m. I think a basement makes sense there.

If reasonably even, then yes and no: 1m below the house footprint would be 50/50 pros and cons for a basement, meaning half of the basement costs fall on the cost centers "construction" and "avoided alternative measures" respectively.

If reasonably even, then yes and no: 1m below the house ground level would be 50/50 pro/con basement, i.e. half of the basement costs are allocated to the cost centers "construction" and "avoided alternative measures" respectively.

Regarding the topography: the property quickly rises about one meter from the sidewalk, then continues to rise evenly by the remaining 2m in elevation until the end of the property. The house will be placed starting 5m from the sidewalk, so it is situated at about 1.5m elevation difference. The neighboring buildings are consistently at least partially basemented. However, during a short on-site visit, the architect stated that a basement is not necessarily required (again, considering the budget issue).

The description of the terrain slope sounds to me almost like an invitation to an UG, among other things for a home office, guest room, and the like. And 1.5 m is, especially from a budget perspective, a 75% "pro basement" vote.

After some time, I had the opportunity to roughly measure the terrain slope myself and ask for an assessment regarding the implementation of the basement.

Here again is a summary of the key data:

[*]Building plot starts at 5m from the street
[*]1m slope is reached after 7.5m from the street
[*]From the building plot (5m), the slope is 50cm within the first 7m
[*]From the building plot (5m), the slope is 20cm within the first 4m
[*]Plot is overall 35m deep
[*]Height difference from street to end of the plot is 3m
[*]Slope from building plot is approximately linear
[*]The house will be about 10m deep from the building plot
[*]The plot slopes continuously upward from the driveway off the street. It is a hillside plot

So we have a terrain rise of max. 70cm within the building plot, depending on how deep the house is. I once received feedback from a BT that a basement is worthwhile as soon as the house is built on a 1m height difference. What is your assessment of the topography?
It is important to us to have as level access as possible from the terrace to the garden or to have stairs leading down. We want to avoid having to retain the terrain or sitting below terrain level on the terrace (especially since the neighbors are significantly higher).

The hip fat does not care whether the neighbors have seen the cream on the cake. My assessment here, rather pro basement, has been confirmed. Does the terrain only rise in depth, i.e., straight sideways? Dry feet are primarily important to me on the terrace. Whoever models the terrain puts on the shoe of drainage responsibility.