Extend the canopy of a prefabricated garage

Or did you mean "ordering a garage that does not yet exist with an even further protruding canopy"? - that would be a completely different matter, as nothing would have to be patched on, but it could be provided like that right from the start.

Then the interior of the garage becomes shorter. 9 meters are allowed.

Is there a building permit for this?

Is there a building permit for it?

My thought was possibly to install a glass canopy.

Only then do you exceed the allowed 9 m.