Base plate built 20cm too high

So I understood it correctly that legal action is my only option, or until then to withhold the €10,000

Construction law is not my thing, so I can't tell you which deadlines already exist by law and when and how and with what notice period you still have to put the contractor in default and after their expiration "threaten" them with this, then alternatively arrange the remedy of the defects yourself.

The only thing certain is: "informal & without notice = fruitless," meaning you can still call a hundred times, and nothing will ever be done. And then you stand there with the ten thousand euros reduction in your pocket and the uncertainty of when you can arrange relief from it. Lawyers don't just argue, they also advise.

From the description, I read: your problem is not yet urgent enough to also be the contractor's problem, so he lets it lie further. If necessary, until the neighborly relationship has a deep scratch from it.

The defect that has arisen is now, of course, the strong height difference over a few meters.

There is no strong height difference. The slope corresponds to 0.2m/8m = 2.5%. It takes at least 1% for rainwater to drain properly at all. Ok, in this case, it initially runs in the wrong direction and you may have to redirect it towards the street, but that is far from being a "strong height difference."

If you had not planned the paving together with your neighbor, you would not have noticed this in 100 years.

So annoying yes, but please don't make a drama out of it. You can pave it slantwise; no car will tip over.

Best regards
Dirk Grafe

There is no significant height difference. The slope corresponds to 0.2m/8m = 2.5%. A 1% slope is needed for rainwater to drain properly at all. Ok, in this case it initially runs in the wrong direction and may then need to be redirected towards the street, but that is far from a "significant height difference".

OK, I probably described it as "not dramatic enough". There is about 8.5m space between the houses. Of that, 5.4m is my garage, which keeps the height level so I can enter the garage. In other words, the 20cm height difference over the approximately 3m to the neighbor will have to be compensated by a slope or steps—about 6.6% or with steps. Less of a problem for me than for my neighbor (my buddy), but visually overall far from "well solved".

more important than the few stones to the neighbor is the legality according to the development plan and co. could you build 20cm higher according to the development plan than originally planned? if a surveyor was already on site and detected the wrong height, he can also say with the development plan whether that is allowed at all. if the fun is allowed, it must be communicated with the office and some paperwork with a few fees will take place. you can probably claim the costs for that from your construction company. if the height is not allowed according to the development plan etc., it gets really interesting. then the cheap stones to the neighbor might quickly become completely unimportant. if the construction company files for bankruptcy, you are left to handle any height changes yourself. a special permit or something similar would be better. whether and what is possible there can be better explained to you by the more knowledgeable people here.

More important than the few stones to the neighbor is the legality according to the development plan and co. Could you build 20cm higher according to the development plan than originally planned?

We built on a plot without a development plan and according to § 34 of the Building Code in a mixed old stock area. The neighbors also have different slab heights. According to the building authority, we could have almost built whatever we wanted, since the environment is so diverse. Primarily, it is about eliminating the pure disadvantages caused by the construction error. Afterwards, I can inquire with the building authority about what they say on this.

wouldn't it be smarter to first ask the building authority whether what is currently there is allowed?! in the end you agree on a few euros for the stones and the authority then says no, but you already signed for the stones and waiver of consequential costs. changing the height would not be reasonable for a finished house and certainly nobody will do that. remains the big question why ~500€ was spent on the distance measurer for squaring. the result here is really quite sad. but hey, better 20cm too high floor than a 2m !!!!! misaligned house like here in the new development area. (or 20cm too low and water stands at the front door whenever it rains...)