Build a terraced end house with an additional unit (GÜ) on your own

We now have the planning documents from the middle house neighbor: it looks like his architect has planned the basement 30cm deeper than our foundation.

That’s offside, even I don’t need video evidence for that

Here in BW there is the notification procedure [...] If I do nothing, the building application is considered approved by us.

The objection is therefore logical – and important because of the Damocles sword of official liability hanging over the approval authority. You thereby protect your neighbor from becoming a perpetrator against your house, which was painfully built anyway, by being too trusting of supposed experts. Sometimes one has to act for the good of all – even at the risk of not everyone being grateful.

Ok I just saw your further post, then get in touch again and ask once more whether he understands that it has to be attempted with the current plan!?

Already done - I don't want to let him walk right into the open knife either.

I also think it is best to remind him of it once more. How much time do you have left for the objection? Since when have the 14 days been running?

Fearing ones, relax!
For the neighbor, this objection "costs" 200 euros for an honorary lap in the acknowledgment procedure and—since he already knows Goalkeeper's objection in substance and can already submit his repaired documents—does not cause much delay. Waiving this objection would be a disservice: the authority would not be warned by explained concerns and would approve, the neighbor’s contractor would build the botch, damage would occur and lead to recourse claims. This is not the beginning of a lasting enmity, but almost a sportsmanlike Christian duty not to let the building rookie run into this ruin.

After several phone calls with our general contractor, the architect of the neighbor, and our neighbor directly, there are some new insights:

Our general contractor clearly states that the basement is even 40cm too deep. Since you cannot simply dig down directly to "Ground Zero" but the foundation must still stand about 10cm "in the sand," this results in the 40cm difference.

I then discussed this with the neighbor's architect. His opinion is that depending on the soil type, you would not have to underpin up to 30cm. He was not even aware of the 10cm "in the sand" standing. That made me a little suspicious. He now knows our position and will talk to the client.

After that, I had another phone call with our neighbor and explained everything to him – among other things, that we will formally file an objection. He didn't find it too bad – he definitely does not want to underpin, so they will probably have to redesign.

On the other hand, our general contractor also informed us that we can probably move into the house from May 11 – good news amid all the hecticness.

His opinion on this is that depending on the type of soil, you would not need to underpin up to 30 cm. He didn't even know about the 10 cm "standing in the sand."

so if the architect didn't know it then he is a loser, or he really doesn't know, but then I would be careful with that, many other things could go wrong. Or he is saving on the underpinning at your expense.