Surface water - Complaint of the neighbors

On the one hand. You have apparently changed a natural slope away from him, now he has water. According to this view, you are responsible for the problem. On the other hand, if you have followed the [B Plan], then you have done exactly what you should do in a development area and have not done anything that you would have to fix. After that, he would be responsible for solving his problem himself. Tricky. Google [Landesbauordnung Niedersachsen Entwässerung]

Here is a photo. On the left, the neighbor's garden, on the right, our driveway!

No no, it doesn't matter. He should have properly allowed his surface water to infiltrate beforehand or directed it to the sewer (and paid for it!) before.

Exciting. We have a similar situation, only that (still) surface water due to clay soil flows from the area above us onto the properties of me and the neighbors. However, this is natural water flow, which I probably even have to tolerate. It is fallow land and will only become building land.

But with you, as you write, it is water from his garage. That alone is enough for him to have to discharge it. That is what the drainage statute prescribes. So you would still have a document. I would advise him to be cautious with his demands.

And the garage has a roof area of about 50sqm. When it rains properly like recently, quite a bit accumulates there.

In the Lower Saxony Building Code of 2003, the topic of rainwater was regulated in §42,2 in a way that was somewhat readable even for laypeople. In the latest version from 2012, it has moved to §41,2 and is described much more unclearly. There, only "wastewater" is mentioned, which then requires a legally watertight explanation of what wastewater is. This is only suitable for a conversation over a beer among lawyers; they are more likely to drink wine...

Just look for the drainage statute of the municipality and go have a beer with the neighbor.