Commitment to the property and what comes next

I’ll answer you here instead of in the provider advertorial thread ;-)

1. Which additional construction costs are not included in the construction and service description?
2. Which applications or permits must be handled by the client?
3. Which preliminary work must be done by the client?
4. Are perimeter insulation and floor construction in the basement included?

1. probably most of them: foundation report up to construction water ...
4. isn’t the offer price from the top of the basement floor (OKKD), so the floor slab is additional?
The price can’t be all-inclusive carefree, and as a general rule for the construction service description: everything that is not explicitly included is extra aka “client-side.”

This is a sloping plot, isn't it?

Then the first step should be to go to the soil engineer and not to (Ver-) Schwörerhaus.

Otherwise, you will be a case for television after one year.

is given as a fixed price by Schwörerhaus

Schwörerhaus is prefabricated construction, then the base plate is added. Possibly also transport, portable toilet, container, and interior finishing. Or is it a shell house?

Apart from that, your entire financing apparently stands and falls with the expected inheritance. With your net income of 3 k€, I already consider it quite ambitious to manage that. In any case, I’m keeping my fingers crossed for you.

The teaser offers usually read quite well, but the devil is in the details.

And in the construction service description (Bauleistungsbeschreibung). ;-)

What have you calculated for the extreme slope? In my opinion, a prefabricated house is not the optimal solution there.