Experiences with border construction?

Hello,

are there any builders here with experience regarding boundary construction? The situation is shown in the attached picture: We could acquire the plot ("our" plot) and would like to build there. To the left is a plot where the house was also built on the property boundary. On "our" plot there is already an old existing house that we would demolish, which was also built on the property boundary to the garden on the right next door. Since "our" plot is only 12m wide in total, we simply cannot leave 3m of space to both property boundaries, as otherwise the house would only be allowed to be 6m wide... The garden on the right next door is standalone, meaning the house to which the garden belongs is two houses away across the street. We have already spoken with the garden owners, and they neither object to us building on the property boundary nor to us having windows on the property boundary (marked in red).

After the holidays, we will try to follow up ourselves with the building authority of the city (the federal state is RLP), but is there anyone here with similar experience? Is the consent declaration with the registration of the building encumbrance on the neighboring property sufficient, or do building authorities tend to make bigger problems?

There is already an old existing house on "our" property, which we would demolish, and which was also built on the property boundary to the garden next door on the right.

Show it (cadastre map and aerial photo).
Your drawing and explanation allow interpretations that it is a §34 area, and possibly there is an obligation to build an extension at least on one side. A design statute or something similar should regulate that. Speculations do not help - and discussions about customary practices would only make sense if there were a development plan.

Oh, sorry - did I forget to mention: According to the city, there is no development plan there. I have attached the excerpt from the Geoportal (red dot/blue arrow would be "our" property) and a picture from Bing Maps. Thanks for the help!

According to the city, there is no development plan there.

I suspected as much. Looking at the probably newest neighboring buildings 38a and 38b, I see the suspicion confirmed that the outlines of the existing buildings have a good chance of being accepted as de facto building envelopes. The situation appears relatively most regulated in the area along Rathenaustraße (north side) and Legienstraße (north side). Maybe you can derive more information there.

Thank you very much for the quick response and assessment! We will approach the municipal building authority as laymen and hope to meet a cooperative clerk there who might be able to provide the official information through a short channel. :)