Neighbor is building a house next to the garage, neighbor's wall is wet

But it's also strange that after 10 years something is leaking or completely normal in older houses! If it's leaking with him, he should take care of it

or how is it regulated with you?

That's exactly the question, why it's happening now. A few years ago, there were already some insects living between the garage and his house, which got through the gaps in his silicone between the walls.

My opinion is also that it's his problem. But he probably wants to shift it onto us. The relationship between him and pretty much all the surrounding neighbors is generally not completely undisturbed. In the coming days, there will be a conversation between him, some craftsman of his, and us, in which I would have liked to go with a bit more ammunition to see if his wall finish shows any obvious defects. It hasn't escalated to the point of involving experts yet. I just wanted to get a third opinion, preferably also from (expert) laypeople.

Then he doesn't need to ask here at all, maybe 1-2 experts here who write something only once every leap year.

ahh! I didn't know that!.

My opinion: If everything is dry for you and the water drains smoothly through your drainage, stay calm and all is well. He built an extension and then took care of the connection between the roof and facade himself. It is, of course, hard for him now to admit that he might have done something wrong. But when I see the amount of silicone, I do not assume that the connection was done correctly. If he really just sealed the gap with the sheet metal and masses of silicone, the error will likely be right there. Silicone is simply not suitable for a permanent seal but must be renewed from time to time. Probably this time has now arrived or is overdue. Correctly, he should have connected to your roof sealing back then and, for example, pulled a roofing membrane up onto his house. Only then the metal strip should have been placed on the roofing membrane to secure it against detachment and similar issues. But this is just my layman's opinion, describing how I would have executed the detail.

But it is also strange that after 10 years something is leaking or

Isn't the cause of the problem due to age?

ahh! I didn't know that!

Even if you are an expert, this is an owners' forum. There are better forums for that, although remote diagnoses are always, to put it mildly, nonsense.

Isn't age the cause of the problem?

I rather assume that the cause of the problem is the incorrect execution of the sealing. However, regarding the amounts of silicone, you are probably right. Age is probably the problem there.