Obtaining documents through a broker - legal question

: That is why I still don't see the seller being obliged to procure or sell the documents. That is the buyer's problem ...

That this cannot be the case becomes clear even with a first consideration:
The building permit is held by the responsible (local) building authority, access is only possible for authorized persons. That means: by proof that one is the owner or partial owner.
Simply going there and saying: "I want to have an insight" is not possible.
Neither for an interested party nor for a buyer of a property.
And whether Patrick has hired a real estate agent or not doesn't matter here and only complicates a matter that doesn't need to be complicated.
So: don't open side conflicts when clarification can be brought to the matter with few reference-oriented words. If Patrick actually had this question on his lips, he would probably ask it.
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KlaRa

That was the question!?!

And development costs do not apply, as far as I know, for a existing building.

Unfortunately, that is not automatically the case: sometimes municipalities realize that they only set a final piece on a street a few years ago and only then "completed" it, and now they could still properly demand contributions from residents, even though from the user's perspective the street has felt finished for ages. I would always have something like that confirmed in writing, that indeed everything has already been completed.

The building permit is held by the responsible (local) building authority, access is only possible for authorized persons. That means: by proof that one is an owner or co-owner.

It is not the person who must be authorized, but the interest. For a prospective buyer, that is the case. You don't have to buy a pig in a poke.

However, one could also include a clause in the purchase contract according to which the seller bindingly assures the condition and is left to make the inquiry.

And development costs do not, to my knowledge, apply to an existing building

Municipalities also like to claim them again after 50-60 years. There are plenty of cases here in NRW. However, you can check with the office whether the property is still affected. In our case, 30 EUR which the seller paid.