Where is a fire door mandatory?

Hello everyone,
my question actually arises from the subject, but to make it a bit clearer:
Attached you will find the floor plan of the basement of our house, which we have purchased. There is still some time before moving in, but we are already giving it some thought.
The heating is located in the room "Keller 1". This room is separated from "Keller 2" by a fire door.
Furthermore, the living area is also separated from "Keller 2" (door by the stairs) by a fire door.

In this case, is the latter fire door mandatory, or could it be replaced by a "normal" door?

Thank you very much for your answers.

Hello

With the output of the heat generator I suspect and the stored fuel quantities, usually none is necessary.


Olli

The gas heating system has 27KW according to my memory. One thing I forgot earlier: From the basement you can go up the stairs to the living room, where a fireplace is installed. Does this circumstance possibly require a fire protection door after all?

The background of my question: We have 2 cats whose litter box is to be placed in the basement. For energy reasons, however, I do not want to remove the door to the basement, but I am considering another door into which a cat flap can then be installed.