What remaining development costs will be incurred by us?

You are probably mistaken about water and electricity. The connection is brought into the house by the supplier, and just because water is already on the property does not automatically mean that no BKZ is incurred.

Okay that means for me that only the costs for the connection from the beginning of the property to the house have to be paid, which should no longer be that high, right?

I have now also called Telekom, the flat rate amount is €799, so I will probably have it done at the same time because the ground does not have to be opened up again.

I just realize that I am hopelessly overwhelmed with this topic :/

Connection costs are usually flat-rate up to 10 or 15m in length.

I will ask the municipality about the whole thing on Tuesday; they should be able to make approximate statements about it.

Thanks to you so far.

About the passage

Umlage der Erschließungkosten zu max. 70 %

can you possibly tell me something about how I should interpret this?

They are allowed to pass on up to 70% of the construction costs for the upstream infrastructure to the property owners as a construction cost surcharge. Often, the construction cost surcharges are more expensive than the actual connection costs, which cover the distance between the house and the line in the street/at the property boundary. The formulas for calculation are occasionally adventurous ;-)