New development area - Are self-borne development costs lawful?

Isn't it common that all of this is already on the property?

No. Fully developed means that everything is at the property. The owner is then later responsible for their property and whatever is installed on it.

Until now, I assumed that with our submitted bid we pay the price for a fully developed property in the end.

Usually, in a new development area, the buyer has to bear the development costs according to Par. x and y.

Resale costs or setting the boundary stones. Isn't that basically already given with a developed property?

No. You buy a property that is approx. xy in size. Nowadays, boundary stones are often no longer set, but only wooden posts. When the house is built, the surveyor comes again (warning: your costs) and measures everything properly.

And what do they mean by costs for house connection (water)... But here one could also interpret that I have to bear the costs from the street to the house.

Yes, of course, it is your property. No one knows where your house will be located.
The utility providers often have flat rates for connection including 15 meters. Regardless of whether your house is 5 or 15 meters away from the supply point, it costs the same.

Costs for the inspection chamber... Sure. But is a cost of almost 5000€ justified?

I even find that cheap.
Mühlacker: we've had that before here in the forum.

Great, many thanks in advance for all the answers. So everything is probably quite usual, that makes me happy.