Ventilation system CO² [ppm] in your rooms

everything that is ventilated.

Yes, so everything, including the overflow areas (to really exclude any misunderstanding).

Good, that means that the corridors are also included since they then serve as overflow areas even if there are no supply or exhaust air valves there. What about a connecting room (airlock) between the living area and the garage, is it also counted as an overflow area or is it excluded?

Is this connecting room inside the thermal envelope? Then yes, that one too. Maybe you could upload your floor plan sometime. Is there already a ventilation concept?