Level the basement or the plot?

The measurement area is the entire property.

So 1.8m slope over almost 40m property depth? That, in turn, is so little that it should hardly influence the question basement - yes/no. If the street were lower now, I would consider the matter of the underground garage, but as it is... have offers made with / without basement and then decide based on weighing costs/benefits.

So 1.8m slope over almost 40m plot depth? That in turn is so little that it should hardly have any influence on the question basement - yes/no. If the street were lower now, I would consider the matter with the underground garage, but as it is... get offers with / without basement and then decide after weighing costs/benefits

Even if I want to bring the ground floor up to street level?

Yes, definitely. Just filling up and compacting layer by layer doesn't cost the earth.

1m³ = approx. 1.5 tons of frost protection/mineral mix.
1 ton of frost protection = max. 20€
can be easily calculated.

Rent an excavator & vibrating plate for 2... 3 days and transport costs for the material depending on the distance. No idea where that 30,000€ should come from, especially since the basic house price usually already includes a certain amount of frost protection and foundation work depending on the provider.

Even with a flat plot, you will have to remove more than just the topsoil for the floor slab.

Regardless of the property
Do you want a basement?
Which rooms should be accommodated there?
There are also builders who build with a basement on a perfectly flat plot and others who fill 2 m or more in the building area so that a basement does not have to be built.

How are your neighboring plots built?

We actually don’t need a basement and would only sensibly put the utility room and technical equipment in the basement. We would still have an office on the ground floor with a view to a bedroom for old age. As a full basement, the basement would also be oversized. The neighboring plot to the north is free, to the east built at a sufficient distance, and to the south the garage is directly on the property boundary with a large house right behind it.

We would still have an office on the ground floor with regard to a bedroom for old age. The cellar would also be oversized as a full basement.

Oh, two approaches for the recommendation at once, here first some more rounds of lots of reading: Especially if 1991 is your birth year, you are more likely to build again near retirement than that it would make sense to prepare a care ground floor. And partial basements are only “saved” in a few special cases.