Raised embankment by neighbors, allowed? What to do?

You can discreetly place a spirit level on his paved area from your property. I think the bigger problem here will be the L-shaped stones, because they seal to a certain extent. Water running down the slope during heavy rain will accumulate there if the clay soil around it is saturated (cohesive).

But let's be honest. You haven't intercepted your property, meaning you ultimately drain onto the neighbor's land. Because you will certainly have a slope away from the house, right? That's not exactly the polite way either. It's annoying that the neighbor now puts a small concrete wall right in front of your nose... but ultimately it's his good right.

He might even demand that you install an eDrainage on your side of the border. I would keep my feet nice and still.

That is not correct, at our place curbstones are concreted in at the border and a privacy fence is installed, I think the curbstones are about 15cm above ground level, so nothing will probably reach the neighbor except in heavy rain.

Our question, is it allowed to simply raise a natural slope?

I want to remind you that you also raised the terrain, namely leveling the basement height. That is no different. According to your drawing, even quite a lot. You should have taken care of your drainage a long time ago... it reads as if you have always drained onto his property.

That is not correct, at our border concrete curbstones are set in and a privacy fence is installed, I think the curbstones are about 15cm above ground level, so apart from heavy rain nothing should reach our neighbor.

And where does your rainwater go at the moment if your soil is clayey? There shouldn’t really be any change if he also takes care of his drainage.

I would like to remind you that you also raised the site, namely leveled the basement height. It is nothing else. According to your drawing, even quite a lot. You should have taken care of your drainage long ago… it reads as if you have always drained onto his property.

I may have drawn it a bit poorly, nothing was raised on our part, but a part of the slope was removed and the house was built into the slope.