Move the basement room into the granny flat?

This regulation has been softened to the extent that it applies not to common rooms, but only to rest/sleeping rooms.

In fact, a distinction is made here between rest/sleeping rooms and common rooms.

This must be stipulated somewhere in your development plan. However, we do not know it either to answer the question.
You will have to provide reasons and sources if the reference possibly constitutes one of the reasons for approval or not.

Yes, this is specified in the development plan under one point as follows:

"Living rooms must be oriented towards the noise-protected side."

I originally had the same problem on the upper floor as well. There I swapped a children's room and an office so that the non-permitted room became an office. That was then approved. So they do actually differentiate.

I received the following justification from the building authority:

"The textual stipulations define that living rooms must be oriented towards the noise-protected side. Based on the justification of the development plan and the noise protection report, the stipulation has already been relaxed insofar as it applies not to living rooms but to sleeping and resting rooms, and as long as at least one window of the affected sleeping or resting room is oriented towards the noise-protected side, the stipulation is considered fulfilled. A further relaxation or exemption from the stipulation is not possible."

Then just turn it into an office as well. It also looks better if the [Einliegerwohnung] ends up being bigger. You will probably want to claim that for tax purposes too. Are there any funding opportunities?

If the tenant then sleeps in there, it's no longer your problem.

Yes exactly, that is my thought. My only question is whether I have to apply for it again if I want to move the door. Or can I just do it like that. The problem is that I have waited an eternity for the approval and now I am finally in the middle of construction. And now pausing again and waiting for the approval would be annoying.

Basically, you are allowed to change a lot inside. A building permit does not regulate your taste or furnishings, but rather that formal rules and guidelines, as well as the external appearance, are observed.

Ok. I am actually torn: Is this still an "innen" modification. On the one hand yes, on the other hand I am turning a basement room into a living space. Maybe someone knows about this legally.