Excavation BKl 4 - only ground slab or entire property?

Hello,
according to the soil survey, we have soil class 4 on a comparatively small plot of 450m2 (the single-family house will have 120m2 of living space).

The soil survey states:

1. Topsoil: humic sands (OH), partially relocated, depth of 0.7 or 0.5m (based on 2 boreholes) below ground level consists of moderately stone-strewn, partially relocated, near-surface strongly compacted topsoil layer.
The topsoil consists, besides organic components, of clayey weakly medium sandy fine sands.
=> Soil class 1

2. Depth at 1.4m: silty sands, soil class 4, soil is not or only conditionally permeable.
3. Depth at 6m: moraine clay (sand-silt-clay mixture, stiff consistency), soil class 4, not permeable.
No groundwater or layer water was encountered in any borehole.

Result:
Topsoil should be replaced with non-cohesive sand or gravel sand to a depth of 0.7 or 0.5m below ground level, possibly deeper.

Now to the question:
According to the civil engineering company, the entire soil on the property should be replaced due to the soil condition.
Our 20-year-old, trained landscape gardener for the exterior area says that often civil engineering companies want to replace everything, but this is not necessary. Rather, it is sufficient if only the area of the foundation slab + 2m around the soil + terrace area are replaced. The rest can almost always be planted.

Who could be more likely right in our individual case?

Just ask the civil engineer why he wants to do that. There is initially little reason for it.

At what elevation will your house be located?

Do you have areas around your property where you can store topsoil? The property is small, so storing it for later fillings would be rather unsuitable.

If you have removed the 50-70 cm of topsoil, I would deliver 20-30 cm of material (sand/gravel depending on the requirement) and then found my base slab on that. Then the finished floor level would be approximately at the height of the previous structure...

On the subject of "soil replacement": Topsoil has to be removed anyway. Are you and the civil engineer agreed on what you mean by that? For you it sounds as if the 2nd layer (sand) should be replaced.

Hi,

the ground outside the house does not need to be replaced. It is not permeable, but that actually doesn’t matter. If there is no layer water there now, it will always run away.

[Bkl4] only concerns the difficulty of solving it.

Best regards

Absolute nonsense and money-grabbing! Let a proper foundation slab be laid and leave the rest. When designing the outdoor facilities, just cover with some topsoil and that's it. The garden doesn’t care about the soil class. And BKL4 means: nothing special, no additional costs!