Pazellenhäuschen without permit, get it approved retrospectively

Conversation, knowledge of human nature, the law regulates a variety of conceivable cases, but does not cover every individual case. That was roughly what was said at the administrative school. Karsten

Laws are not made for individual cases. For that reason, they are deliberately kept abstract and interpreted by the practitioner. The principle of proportionality and discretion exist for good reason.

Exactly, because the law does not cover every individual case, but life is concrete and individual, there is room for discretion. One just has to dare to use it. You are always on the safe side only with rules are rules and period.

And if the only way to legalize is the "relocation" of a file, one very quickly ends up in corruption/nepotism and thus fosters the lack of trust in the state.

There are quite a few answers coming in...

Thanks for that already!
So my intention is not necessarily to make a legal little house out of it. It would be enough for me if within the next 2-3 years no one comes to my yard telling me that I have to tear the whole thing down and dispose of it at my own expense.
As I said, the little house has a foundation, which would probably also have to be removed.

The question would also be what the possible penalty could be, i.e. just demolition? Or even a fine? Back payments on property taxes? The authorities will surely come up with something.

Well, whatever. For the 2-3 years I would actually want to insulate it inside on a low-budget basis.
Roofing felt is also not particularly expensive, and you can often get wood for the repairs for free.

A question on the side:
Theoretically, it would also be okay for me to set up a caravan including a porch tent there, in case worst-case it comes to the point where the house has to go.
Now the question is what requires approval.
Can I pour a small "foundation" for a barbecue area?
For example, 50cm x 50cm including grill on top?
A small garden pond?

If large sums come in after finishing my studies, I would still like to live there ;).
Including renovation etc.
I really like the whole thing because of its sentimental value and it's nice and quiet there as well. It would actually be a shame if the whole thing fails one day.

Was the house ever permanently inhabited? Is it supplied with electricity/gas/water etc.? Has it been disposed of? Wastewater? Trash? Did it have a number? Were the residents officially registered there? Did mail go there? Also official mail? .....Then talk to the responsible municipality. Karsten