Are the costs justified?

Nearly 30 k€ for earthworks without a basement is still quite a sum, we didn’t even have that much with a basement... Nevertheless, I thought that was the surcharge for the 50 cm deeper excavation...

what if the development plan allows a full 37cm higher? normally, the plinth heights from a certain fixed point are relatively precisely specified (with us 0 to 50cm above street level at a certain spot).

the height of the plinth (or finished ground floor level) is determined during alignment. the builder says what height they would like within the allowed range. the advantages/disadvantages are explained, in the best case, by the experts present who may also give a tip to negotiate the height with the neighbor somehow.
the civil engineer must dig down until a suitable soil foundation is found for the load-bearing capacity of the house (e.g., natural earth). it does not matter at all how high the house will be built later. the higher the house is later in relation to the street, the more filling will naturally be needed. this means the house will flood later in case of an overflow.

the higher you build, the less stress there is later with rainwater. the higher you build, the more expensive it becomes. from certain height differences to the neighbor, expensive L-angles may be required as a boundary (roughly from 40cm height difference, as then a washed concrete slab/similar is no longer possible).

Such differences in elevation can only be dreamed of in many places in the south.
I would definitely also make sure to be at least 15 - 20 cm above the street anyway due to possible flooding.

if the development plan allows a full 37cm higher? normally, the base heights from a specific fixed point are relatively precisely defined (with us 0 to 50cm above street level at a certain point).

Since no one can yet assess the (error) source of the 37cm, this is purely hypothetical. However, the standard requirements in development plans vary regionally. If it is technically necessary, it is no drama.

the height of the base (or finished ground floor) is determined during staking out. the builder says what he would like within the permitted height.

If it was done that way, that would of course explain why suddenly 37cm are missing. If a builder on site tells me he wants it different than in the building application, he signs it on site and it is then also recorded in the staking out sketch.

Correct is probably rather that one adheres to the building application – otherwise the setback areas might not fit.

the pros and cons are explained at best by the experts present and may possibly give a tip on how to somehow achieve the height compared to the neighbor.

Guessing is unfortunately very bad there.

Best regards
Dirk Grafe

The height of the finished ground floor level on the ground floor (which is ultimately what it's about) was determined at our measurement appointment. The civil engineer, the construction manager, the surveyors, and we as the builders were on site. The surveyor read from the development plan, stated the height we had at the measurement point (boundary stone), and from where to where we could choose the height. Then we went to see how it looked on the neighboring properties and the street. Afterwards, there was a brief discussion about which height would probably be best. (It was 35cm out of the allowed 50 - which is already quite high and now caused some additional costs due to more gravel, etc. - but the whole property is a good bit higher than the rest). The height was not part of the building notification at all. It was only required to comply with the heights specified in the development plan (0 to 50cm above street level). Due to the permitted ridge height of 10.5 meters, with a city villa just under 7 meters high, no problems are to be expected. It may be different in other areas/federal states. For us, it was just that simple and uncomplicated. How should one know from the building notification which height would be best in the end? If you build too low (because indicated so before), water runs in during every heavy rain.