Planning a roof extension - remodeling with architect, structural engineer, building authority?

Good day,

first of all, "hello" to everyone.
I registered here because we are currently planning to add a floor to a house to create an apartment.

Now we are a bit at the beginning of the planning to see if we can get approval etc.
Since this area is completely new to me and I can't really find anything meaningful on the internet, I wanted to ask here.

Can someone explain to me the procedure and how I should proceed here?

- Architect, structural engineer, building authority?
Who ultimately approves this remodeling measure for me?

Thank you very much,
Outcore

First clarify the approval feasibility: find out whether you would be allowed to newly build the "After" house. If the development plan says "no" to that, then this will also apply to a remodeling result.