Can I simply change the architect? I assume I have to pay for his previous work, i.e. the first draft, even if we don’t like it. I just hope he doesn’t charge extra because he now has to start from scratch.
Just because you mention it and should check it for yourself doesn’t mean you have to change right away ;) Does he have references? Ultimately, of course, you also have to trust him if you yourself don’t understand the development plan or lack the knowledge of what options you have. I just wanted to mention it because with split level you also get ceiling height. I would ask him everything that you get here as brainstorming.
Is it that simple? Wouldn’t there have to be a 6 m distance maintained, because the neighbor is on the boundary and then basically his setback area has to be taken over on the own property?
Don’t know. The original house is being rebuilt anyway. You can also declare an inner courtyard so that it counts as part of the house. Or you make a clear L.

It is a small settlement area from the 1930s where they nowadays apparently approve quite a bit freely. Hamburg is happy when living space is generated. The red is the building envelope, then there is this dashed line for “other buildings.” Possibly the conservatory can be positioned here too if it is marketed cleverly. It can be projected somewhat in front of the house with a 3 meter height – it would also only be an annex of a gardening nature ;)