Build on the property boundary

Well, I'll think further now: I doubt that an objection will help you. The result will be that your neighbor builds a 9-meter carport and then also a garden shed behind it. He is allowed to do that and in the end you're in the same position as with the carport. How long is the carport supposed to be?

I don't know if he's really allowed to do that. I don't know how it is in Hamburg, but here in Lower Saxony I am allowed to build up to a maximum of 9m towards a boundary line.

The limit of 9m also exists here in the [Landesbauordnung]

Normally he is not allowed to do that. In the building regulations it is usually 9 meters in a row, and a total of 15 meters (if he already has 10 meters at the other neighbor, he may only build 5 meters at your side). However, there are development plans in which more or less border construction is permitted. Therefore, I would first look at the development plan - then at the building regulations. Afterwards, talk to the neighbor and inform him that, from your point of view, he is not allowed to build more than legally permitted. If he wants to build more, he needs an exception permit from the building supervisory authority - I believe you can then file an objection as a neighbor if he receives a permit for more construction. If he builds without a permit, you can report the illegal construction to the building supervisory authority and sue for dismantling,...

So without your consent, it doesn't work anyway.

I don't know if he is really allowed to do that. I don't know how it is in Hamburg, but here in Lower Saxony I am allowed to build a maximum of 9m at a boundary.

That's correct. I was assuming Rhineland-Palatinate. There, 12 meters may be built on one side or 18m on all sides. In Hamburg, it is actually only 9m per side. That looks bad for the neighbor with his carport.

Without reason? I don't know what your relationship with each other is like, but maybe he just wanted to be nice at first and tap on the door to see what you think about it.

Do you have a particularly beautiful view of the Elbe in the last three meters there?