Building permit for a single-family house granted, is an additional granny flat still possible?

Assume the apartment is to be rented out. Then there should also be some possibility to deduct something from taxes that does not exist with ownership. Loan conditions could also turn out to be better, mind you, could.

In the basement kitchen connections and toilet. Separate entrance is available. Ceiling height 2.32m. ... light well and shaft already marked. One more partition wall to our area.... nothing more....

Minimum height 2.40
Separately controllable heating
Separately controllable [Kontrollierte-Wohnraumlüftung]
Minimum area of windows, etc.

Minimum height 2.40
Separately controllable heating
Separately controllable controlled residential ventilation
Minimum area of windows etc.

This is not a requirement

Who should oppose if these things are not met?
I consider that to be incorrect

What you think of it and what is fact does not matter to me.

Living space must have a room height of 2.4 meters.

just like the rest.

That is not a requirement

Who should oppose it if these things are not met?
I consider that to be incorrect

Rooms used for living are only such if certain conditions, which are set in the state building code, are met. Privately you can ignore it, but not when renting out.

Minimum height 2.40

BW 2.30... take another look.

State Building Code for Baden-Württemberg
(State Building Code) 1
as of March 5, 2010
§ 34
Rooms used for living
(1) The clear height of rooms used for living must be at least:

1.
2.2 m over at least half of their floor area, if the rooms are wholly or predominantly in the roof space; parts of the room with a clear height up to 1.5 m are disregarded,

2.
2.3 m in all other cases.

So it fits.

Even though I can't imagine that from just an idea, simply building a low wall, an accessory apartment could arise.