The problem lies elsewhere. The developer acquired a plot of land and planned a house for a fixed price on it and sold it. Then the building application is submitted, and it doesn't work without special regulations. There was obviously no consultation with the client either, but the client incurred costs out of the blue or these costs were included in the planning for the approval.
Developers/architects must know that such things can happen, especially with narrow plots, and they must include this in their cost calculation as a mixed calculation. Passing this on to the client without consultation, if the client has not ordered extra changes that trigger these costs, is, in my opinion, neither legally nor morally acceptable.
Moreover, this may possibly be a planning error by the architect, against which he is insured. However, the damage is so small that it is below the deductible, so it cannot be reported to the insurer. Therefore, an attempt is made to collect it from the client.
As I said - if this happens to me, I have to be able to prove in writing that I informed the client about the possible costs, otherwise the charge notice goes to me (!) and I am also liable for the costs. I would also get the original notices - if the developer is named as the cost bearer there...
Best regards Dirk Grafe