Non-compliance with the development plan by the neighbor

They probably do little to nothing on their own?

Yes.

Even if I gave them a hint.

Yes.

In doubt, probably only an official complaint to the building authority would have any effect

Probably not.

Besides that, I wonder how likely it would be that he would have to dismantle everything then.

If one decides to intervene, a dismantling order is very likely.

I fear that in the end he would only have to pay a fine and everything would stay as it is anyway and I would have trouble forever.

I don't believe that. A terrace is not such a valuable structure that dismantling would seem disproportionate.

But he doesn't get to the point and they have already been living in the new house for a year.

That is annoying, but with a new building it is not such a long time to become unfriendly.

Take into account 's suggestion from #14 and everything will be fine. You have already indicated that you are not looking for a dispute. And what good is dismantling if something similarly annoying is then built in a permitted place.

Well, if the neighbor is not reasonable, just place the garden shed right on the boundary, right in front of their nose. If the circumstances allow it. Or isn't there still a compost heap missing? Or an insect hotel, or or

I don't have the locations in mind... But anyone who is that cheeky doesn't deserve any better.

Dismantling does not necessarily have to be the consequence, the neighbor can also apply for an exemption. The Lower Saxony Building Code 2018 and also the Baunutzungsverordnung do not contradict this. An embankment would indeed be a structural facility, but it is exempt from a boundary distance. NBau §5 Abs.8(1). A structural facility is distinguished from a building. NBau §2 Abs.1 (5) Baunutzungsverordnung §14 does not refer to terraces. It would only be an exemption according to the development plan, so far I have never had problems obtaining an exemption. Because the question is, why does this passage even exist in the development plan. It could also be to protect the owners in case of flooding, if such is to be expected in the region. Of course, it can also be a nature conservation or water protection area. Depending on the goal of the B-Plan, I would be cautious.

Our nice neighbor has built a relatively large terrace, about 0.5m elevated, exactly in this area, right on our property boundary (detached from the house).

... our nice...

I was not asked about it

The whole thing really bothers me a lot because it pushes so close on me. (The whole thing is less than 5m in a straight line to our main terrace; His actual terrace is on the other side of the property)

Since no fence has been built yet (which he is responsible for, because it’s the right side) you can also see through half of the ground floor from this terrace.
I don’t find that very pleasant either.

A fence also does not protect against curious looks

I am 100% sure

O-o

Follow @kaho674’s suggestion from #14 and everything will be fine. You have already indicated that you are not looking for a fight.

I don’t see it that way.

You’re pretty pissed: first he doesn’t fulfill his duties and doesn’t build a fence, now also a terrace in the fringe development area.
Even with (his) fence, each of you could see the other in the garden. You are responsible for your own garden and could have planted bushes and hedges long ago. Because everyone is responsible for their own privacy screen. Then he couldn’t look straight through your house from the terrace now either.
How should one imagine that? Is he now sitting there with his family by the fence looking over at you?
I find the whole situation a bit surreal: after all, you are old enough to address and resolve a grievance. And as already said: you are responsible for establishing your own privacy screen. If you had taken care of that, you wouldn’t even see the terrace.

And yes, everyone has a somewhat different understanding of “auxiliary facilities”. Maybe he doesn’t even know the clause? Not everyone knows the development plan by heart, not everyone is willing to deal with it. That’s what you have neighbors for, who could point it out to you (instead of going straight to the office).
But I think things are escalating a bit, the reason lies in the past and doesn’t want to be cleared away, right?
I’d be interested in a drawing where the measurements and distances can be clearly seen. Would you like to sketch that?

I am somewhat in a dilemma. On the one hand, I have no desire for a legal dispute, but on the other hand, I don't have to put up with everything.

I asked the municipality myself without snitching. I pretended as if I wanted to build a terrace at the back of my property. The statement was that it is an auxiliary structure and I am not allowed to do that....

How am I supposed to conjure up a 3-meter hedge in 1.5 years? Besides, it's nice that he builds such nonsense on the boundary and I am supposed to figure out how to protect myself?

If they sit there, it's almost as if they are sitting on my terrace with me..... at the moment, there is a small above-ground pool on it.... which is also great....

I can make a sketch later.

Yes please sketch... I've already given tips for escalation