Procedure for purchasing a property in need of renovation

So for something like an elevation, to my knowledge, you would definitely have to obtain a building permit because of the static calculations? They would have to be available. For the conversion, you look into the building regulations of your federal state, regarding attic conversions and also escape routes.

Where do I ask if it is allowed as living space or if it could still be done retrospectively (probably useful for insurance purposes)?

For that you will need more qualified documents than just a sales exposé: approved is the living space only with the approval; it is approvable if the requirements are met (e.g. minimum room heights, escape routes), and the residential use can be allowed, as explained in post #15, possibly even as an illegal building (which is not the same as a building permit).

As far as I know, you would definitely have to obtain a building permit for something like an elevation because of the structural engineering? That would have to be available. For the expansion, you look at the building regulations of your federal state regarding attic conversions and also escape routes.

It is probably available.
Then I'll start looking for the building regulations (although the real estate agent should know that too).

Thank you very much.

Realtors want to sell houses quickly and for a high price. They don’t have to know anything. In a seller’s market, they sell the house anyway. Unfortunately, that’s how it is; one shouldn’t put much stock in statements from realtors. By the way, we had a similar problem and noticed after the purchase that our attic did not meet the requirements for living space in one respect.

Brief update (for those who are interested): Due to high demand, the house will go into a bidding process. We have immediately stepped back. Approval as living space would be feasible, but the staircase (quite steep) would probably have to be altered extensively and the carport next to the house must also go. Then there are a few more things that were built as "Schwarzbau" and some renovations in the next 5 years. Definitely too expensive for us...

We immediately distanced ourselves. [...] Then a few more things were added that were created as an "illegal construction"

Yes, bidding on a partial illegal construction counts for me under the category "never catch a falling knife" ;-)