Secondary construction costs - Need your expertise

Hello dear community,

I would like to get your opinion. The following case:
We want to build a house. The plot is about 250 sqm, in Koblenz (over 100 inhabitants).
There is a two-story house (an old building) on the plot, measuring about 6 * 10 meters. Below it is a vaulted cellar. This old building is to be demolished, the cellar "sealed," or "filled up" (I don't know the technical term for this). Afterwards, a new house (solid construction) with the dimensions of about 8*10 meters is to be built on it. There should be no new cellar. The old building is very close to another building (which is also basemented), but the two buildings do not touch.

Since this is a flood area, a kind of cellar construction is to be created on the ground floor, but no real cellar. Instead, parking spaces for our cars are to be created here. The living units are to be on the first and second floors.
We are getting the property as a gift. We only have to cover the costs for the notary.

Now to my specific question:
A regional house construction provider (Kern-Haus) estimates the additional costs at around 80K. These do not include the costs for the notary, nor the costs for the yard (no green areas or garden are planned).
However, among other things, the costs for the demolition and the costs for filling in the cellar are to be included. There is definitely to be no own contribution, except for the demolition. The actual house we want to cost 300-320K.

Are the 80K realistic, set too high, or too low? Do you need more information?

Thank you in advance.
Best regards, Ralf

Hello Ralf, unfortunately you will not receive a correct answer to this question. Usually only laypeople are involved here (with very few exceptions) or builders who have received this or that answer from the contractor. Regardless, you should try to obtain several offers specifically for this filling (or whatever it is called). The answer will be somewhere around one offer +/-10%.

Hello Ralf, Without knowing the construction project in detail, it is difficult to estimate the additional costs. As a rule, I assume 17-21% of the net construction sum for the first cost estimate. Now we have to take into account additional effort for controlled partial demolition and backfilling with you. Good luck Ralph

Thank you for the feedback. A quick (perhaps stupid) question: what is the Nettobausumme?

There are no stupid questions. The architects' fees are determined, for example, based on the net construction sum. All construction costs from the foundation to the shell construction, technical installation such as heating, ventilation, sanitation, as well as plastering, painting, and flooring work, etc. without VAT

Hello Ralf, "Nebenkosten" is not a fixed term. Your provider, for example, includes demolition and disposal in it. It is completely impossible for me to estimate whether the estimate is good or way off. There must be a reason why the provider does not offer a fixed price: he has to estimate himself. Here you probably have to compare something with the concrete circumstances.