Renovation of multi-generational house - bad feeling with general contractor

In my assumption, the house is attached to the neighboring house on the right side as a semi-detached house (?), but not a terraced house (?) and therefore must maintain a boundary distance to the neighbor on the left side (depending on the federal state, 3.0 or 2.5 meters or more). However, here the entire previous 5.4 meters boundary distance is closed with living rooms (which are not boundary privileged).

The residential area is a terraced housing development. Our architect argued as follows about the 2.5m boundary distance:

The idea [behind the new staircase] is that you basically set the main house at a minimum boundary distance of 2.50 m to the neighboring building and only the staircase house is directly attached.

That sounded very plausible to me at the time, but after your post I am no longer so sure about it.

How many parking spaces are required for you? I see two, but often 1.5 parking spaces per residential unit are required. Why don't you shift the carport a bit to the right? As it is now, and if two cars are parked there, you can no longer get to the garden gate.

Does the neighbor actually have a basement? If not, then it will be tricky with the support.

The issue with the boundary distance mentioned by 11ant has also caught my eye. Has this already been clarified with the building authority?

I thought two parking spaces per residential unit, I’ll have to check. Thanks for the hints!

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We will now first make it clear to our general contractor that we unfortunately cannot proceed without a rough cost framework (thanks to K1300S).
Then we will wait to see if anything comes from him; otherwise, we will probably have to look for alternatives in parallel.

Thank you all for the remarks on feasibility, etc. This has already helped us a lot!

The residential complex is a terraced housing development. Our architect argued the following regarding the 2.5m boundary distance: The idea [behind the new staircase] is that you basically set the main house at a minimum boundary distance of 2.50 m from the neighboring building and only the staircase is attached directly. That sounded quite plausible to me at the time, but after your post, I'm no longer so sure.

Either it is a terraced housing development (g) with a corresponding obligation to build on both sides or not (o) with boundary distance. There is no "I don’t know" in development plans.

We have now spent almost €11,000 on the architect's plans without knowing whether the entire renovation project is even feasible for us.

What exactly did you commission for €11,000, or how is the billing done? At this rate, you will need another 6 months and another €10,000 until the building permit. Which federal state is this? At least in NRW, you can also build a dormer with well under 2.5m or 3m distance to the neighbor and thus gain more space. Maybe it’s time to draw a line and look for an independent architect. I could not continue working with the general contractor and his wife.