Move the basement room into the granny flat?

I don't know if you read my question correctly (it's even in the title), but that's exactly what it's about.

To be honest, the headline falls out of the window area when you go through the message function and a lot has already been written anyway. Then you go through the entire content of the discussion and refer to the new comment.
In this respect:

Basically, a lot can be changed inside. A building permit does not regulate your taste or furnishing, but that formal rules and guidelines, as well as the external appearance, are complied with.

can the noise problem be solved with a soundproof window? I think the building authority doesn't give a damn where the door is. Problems could only be caused by the tenant if he pays the full price per square meter for an official basement room. I left out one window and made one smaller, the building inspection didn't care. But normally no one comes there

can the noise issue be solved with a soundproof window?

Soundproof windows are mandatory and are also installed. But nevertheless, it may not be a bedroom.

Problems could only arise if the tenant pays the full price per m² for an official basement room.

Regarding the tenant: After the change, it would actually no longer be a basement room in reality, but a hobby room or workspace. Am I then not allowed to declare it as such when renting it out, because it was declared as a basement room in the building application? The construction does not change, the only thing that would change is the designation if I am allowed to do so. When renting out, I would like to indicate that it is such a corresponding room.

Am I then not allowed to declare it that way when renting out,

Of course, you are allowed to declare "plus a 12 sqm hobby room".