If you divide the plot along the shared wall so that each house is on its own plot, which is at least 350 sqm in size, I no longer see any reason against your planning.
You are also not allowed to build, on the one hand, a one-story house – which can actually be designed as a two-story house in terms of calculations – and as an extension an original one-story house – i.e., with a flat roof.
I don’t find any reference to this in the development plan excerpts. Maybe you can briefly explain why this should be the case. Are there regulations stating that the halves of a semi-detached house must look identical?
A nice lady from the local office told me that terraced housing would even be possible on the plot but not a detached house with two residential units????
I assume that the boundaries of your plot coincide with the street boundary lines and usage delimitations within the red marking. Accordingly, your plot includes a complete building field where up to three residential units are possible. But only as a housing group. So you could also divide your plot into three plots of at least 300 sqm each and, for example, build three terraced houses.
The development plan apparently aims to restrict settlement density by limiting the residential units
per plot and at the same time setting minimum plot sizes. Here in the Rhineland, without a development plan, you would probably build at least six terraced houses on the plot, which the city planners want to prevent with their regulations in your case.