ᐅ Basement Design for KfW 55 Standard + Timber Prefabricated House

Hello everyone, regarding the utility basement, I have received different opinions. Some say that the basement stairway must be separated by a wall with a door, others say the stairway can be left open and only the basement anteroom needs to be heated, and others say that heating is not necessary at all.
What is actually correct here?

Thanks for your help.

We installed a stud wall with insulation in the basement entrance area. A vapor retarder was also added on the exterior wall. The doors to the unheated rooms are built to Thermal Class III. This entrance area is heated by the open stairwell. This is the standard procedure used by our builder.

When we inquired about having a utility basement, we were told that a door to the basement is required because we are building to KfW55 standards.