Sauna in the garden in Bavaria - distance to neighboring property

I just checked, there is nothing about it in the [Brandenburger Garagenverordnung].

In your state building code the term "Garage". The terminology is the regulation. You will find nothing because the term garage, just like ancillary facility, is clearly defined. There is no paragraph that refers to something that does not exist.

Sorry, but then the term Garage would have to be clearly defined in the regulation or in another law. Where do you find that?

I can also gladly share a photo with the view of the property.

Better would be a site plan with all planned buildings and the development plan with all available documents. If the latter is available online, a reference - no link! - to the location (municipality, development plan name or number).

Now to my question, can/may a small sauna house stand on the edge of the property next to the neighbor?

That depends ... see above.

And may the sauna be located beyond the building boundary according to the development plan?

That depends ... see above.

A sauna is indeed a room for staying

That depends ... for example, whether it includes a relaxation room.

You won’t get a permit for that

Up to 75m³ this is also not required.

A garage party would then also be prohibited, if the garage is on the property boundary?

That is correct. Except that usually no one is on duty at party time who could initiate a building code procedure. And by Monday the garage will have been restored to its original condition, so a prohibition of use would be ineffective.

Sorry, but then the term garage would have to be clearly defined in the regulation or in another law.

It is, at least with the clarity one can expect from a law.

Where do you find that?

§2, paragraph 7, BbgBO
Garages are buildings or parts of buildings intended for parking motor vehicles. Exhibition, sales, workshop, and storage rooms for motor vehicles are not parking spaces or garages.

... It depends ... for example, whether it includes a relaxation room. ...

So the actual lounge, meaning the room where the stove is, is not a lounge? Or in this case, is the relaxation room actually the lounge?

Then the actual lounge, that is the room where the stove is located, is not a lounge?

Whereas the definition of a lounge is apparently disputed.

From everything I found in my research, the sauna is in a borderline area regarding the lounge, a gray area that has not yet been legally clarified.

I once read a court ruling, which I of course can no longer find, stating that a single-room sauna is not a lounge and is therefore privileged under distance regulations. BayBO, Art. 2, paragraph 5 (5) Aufenthaltsräume sind Räume, die zum nicht nur vorübergehenden Aufenthalt von Menschen bestimmt oder geeignet sind. It is in the nature of the sauna that the heated room is only used temporarily, just as is the case with the toilet.