Kfw 458 - Heating system replacement - Disposal proof?

Which authority did that come from and is a legal basis named in the letter? This must exist and also be comprehensible to you.

From the district administration, department of water and soil protection.
Legal basis Paragraph 1, Abs. 2 sentence 3 of the Ordinance on Installations for Handling Water-Hazardous Substances in conjunction with Paragraph 23 of the Hessian Installations Ordinance.
The inspection report then states "Inspection of a heating oil consumer installation according to VAwS."
From the documents I could also see that we are classified as above ground and only in Drinking Water Protection Zone 3.

I am currently getting the impression that something is going wrong here. The legal basis does not make sense to me in relation to our situation. It is always about underground facilities or over 10,000 liters, both of which do not apply, because the protocol states "above ground".

that we are considered above ground

Therefore, no ongoing inspection requirements apply.

only drinking water protection zone 3

I am not specifically familiar with Hesse. But in my opinion, you are in a protected area and thus the 1,000-liter limit applies. At least, that is generally the case.

But with that, in my opinion, you are in a protected area and thus the 1,000 liter limit applies. At least, that is generally the case.

Ok. Here, all houses that are not located somewhere outside the towns are at least in protection zone 3, because the zones each cover at least the area of the villages. But then that is probably the reason.