Caution with the height. I think you will already have difficulties here... I believe you are not allowed to go 3m higher. And yes, it counts as [Grenzbebauung].
Thank you for the assessment. We only reach about 2.5m from the GG boundary at 3m, so that should not be a problem. We are more concerned about the length of the boundary construction...
With us, it is such that the double carport may be built on the property boundary and is even exempt from building application if under 40 sqm. That means approximately 6x6.5m.