Distance from terrace to property boundary

Before this escalates, deal with the building authority in a factual manner. Your neighbor remains your neighbor. Do you want a war even before you have moved in?

The building authority does nothing because it is a civil matter. The terrace with the small distance is apparently allowed, although (also) outside the building envelope. In the building application, an exceedance of the building boundary due to the terrace of just under 2m was requested.

We have already moved in and live here... the "war" is already happening. But also with other neighbors, but that is another matter.

I might as well build a townhouse.

With the positive difference that everyone looks in the same direction and not at each other's plate ;)

the "war" is already here.

So that was the heart of the matter. You are looking for weapons. He who seeks, finds, but remember, in war only those standing outside the battlefield win.

So that was the heart of the matter. You are searching for weapons.

For God's sake, no. I just want it to be done correctly and not botched, that's all. And if he thinks he is right, which he might not be, I need something to be able to show him. Like what e.g. wrote about the slope and the proximity to the border. And as a layman, you don't have that immediately on your radar.

Heavy rainwater flows exactly onto your property. It is not allowed to do that.

Unfortunately, that is not entirely correct.
If the original terrain already had a slope so that the water flows to the neighbor, and nothing is changed about the slope, then one is not obliged to retain the water on one’s own property.
Only if one makes changes to the original topography is one responsible for ensuring that the change does not disadvantage the neighbors.

Deviating from this, of course there can also be specific regulations regarding this in the development plan.

Did the neighbor build after the building application?

They often build a few centimeters higher and then make the terrace level with the ground.

The street has a slope of 0.5%. Your property as well.

With an estimated house distance center to center of about 20 m * 0.5% = the height difference should be 10 cm.

What does the development plan specify for the finished floor height of the ground floor?