Building plot: Massive fill or better basement with excavation?

"With 2 floors + roof, they earn more than with a single-story building placed on a residential basement" I have come to suspect this as well. However, the clay soil costs €25/m³ plus transport at the landfill. Not cheap. The development plan also stipulates that the eaves height from street level may be a maximum of 4.75 m. Two full stories with completely straight walls also in the upper floor could be difficult there. Or?

No, you cannot have 2 full floors at the eave height.

There are timber frame builders who plan with solid underground floors.
Those we spoke with leaned towards tower construction, garage, and storage cellar under the house or retaining wall and the house with 1 meter distance in front.

For 25 euros per cubic meter, you won't get any fill material delivered and installed.

We have solid wood
In the basement, the hillside wall is made of cast-in-place concrete, insulated on both sides, and wood on 3 sides. The hillside wall was expensive.

I can't imagine that filling, retaining, outdoor facilities + 2 full floors (which you can't build) is cheaper than
basement, ground floor, and attic with corresponding outdoor facilities.

Would you even get that approved?

Refilling would not be a problem according to the building authority. I myself am surprised about this civil engineer. However, he is considered very experienced.

What construction costs per sqm of living space turnkey with normal equipment are generally common today?

Talk to other companies

Price
Rule of thumb 2,000 euros per sqm of living space (in BW it is the Energy Saving Ordinance and discipline in the equipment)
Ancillary building costs
Outdoor area (this includes terrain modeling, retaining wall, etc.)
Earthworks

Normally, planning is done so that as little excavation as possible is required or filling must be done.