Non-compliance with the development plan by the neighbor

Off the cuff, there is no best solution for me.

Everything has its advantages and disadvantages, with the disadvantages sometimes outweighing.

Besides, I don’t want to complain here. I just don’t think it’s great when it is generalized that I am the problem and only looking for trouble. I have done nothing unlawful.

So you do absolutely nothing and swallow the anger inside you.

even other neighbors ask me why he is building something like that on the border or if he is allowed to......

Then it would only be a matter of time before the other neighbors officially complain and the authorities take action. The neighbor will point the finger at you.

Well, talk to your neighbor... it won’t get any better.
You can forget about the garden shed because if your immediate neighbor doesn’t snitch on you, then someone else will.

I just don't know if someone who is not directly affected (because they live 5 houses down) would rat someone out. From talk like "is he actually allowed to do that" to taking action... I don't know. To be honest, I also don't know what I should address there, since my opinion is not of interest anyway.

There is no best solution for me off the top of my head.

Yes, it's not easy. A terrace like that also costs quite a bit. I would prepare a good text and then politely approach the neighbors about it. In such a delicate matter, always offer several solutions so that the other party doesn't stubbornly shut down out of desperation. But it won't work without talking.

Just go to your neighbor when he’s sitting on the terrace again and talk to him.
Look, I understand that you built a terrace there, great view. But look, you can see through all my rooms up to the bathroom door, and that makes me feel uncomfortable.
Now comes a horrified look – genuine or played – from the neighbor and an Oh, I didn’t even notice that.
Then you ask him what he thinks about a trellis with clematis growing on it (or whatever you’d like there) and if he’d help you put it up. That way his privacy is also ensured again.

With the garden shed, it might have to go again tomorrow. I would have decided on a nice, cheap privacy screen.

But this is already a more serious cut to my quality of living, which theo. is not allowed. And that makes me angry.

Yes, then go your way to the building authority. Or wherever. Before you get really upset about doing nothing, then file your complaint. I actually don’t understand what you want from us now: a question, several answers. And you’re still grumbling and not taking your steps. I am slowly getting the feeling that we are all going to be blamed here for you feeling uncomfortable on your property, just because the majority tells you that only ONE step changes something, namely the one to your neighbor! Tell him what you write here. You can say something like that politely. Soon I’ll get a boo again for suggesting to say it to him politely

Make YOUR move, file your complaint if you don’t want to talk to your neighbor. But then that really is your problem.

If someone builds something disturbing and unauthorized on the common boundary without properly informing the neighbor beforehand and does whatever he wants. The neighbor’s opinion is therefore irrelevant and no consideration is taken. Then I ask myself who is looking for or provoking the dispute?

But don’t forget: he also has the right to move on his property, to stand at the edge of the property, to sit, whatever... that works just fine without the terrace. You don’t have to stick to the house, neither do you. Move your butt over to him, take two bottles of beer, and that’s that....