Build the garage yourself or not? What is cheaper?

Is no building permit required at all?

It is probably exempt from approval, the house was too

For the installations according to sentence 1 numbers 1 to 4, the border-side average wall height above ground level must not be higher than 3 m and the area of these walls at each neighboring boundary must not exceed 25 m² in total

What wall height is planned?

Not higher than 3m, but it is also at the very beginning, about 255-270cm high

Yes, not higher than 3m, but not more than 25 sqm area are not mutually exclusive. One can also choose 8m length at 3m height.

With the planned height, one should precisely clarify from where the measurement is taken.

And, sorry, if one asks a question that has already been answered .. one does not always have all the facts in mind about projects that are not their own.

That is an important point.
If your wall on the border is 9.5m long, it may be a maximum of 2.6315 m high. Otherwise, you exceed the maximum allowed 25m².
I don’t know whether the wall stops before the roof, or if the roof structure of presumably 30-50cm must be included.

And "is <> approval-free"
clarify that, "wohl" is not enough.
If the neighbor complains, it will be bad. And he only starts when you are already finished.

Everything that is explicitly approved causes no more trouble.