Build monolithic according to KfW55 standard with a basement, but it makes sense!

That really has to be calculated. Remote diagnosis is impossible. The energy consultant is one of the next stops anyway. What is stopping you from starting with that?

Besides the fact that the surveying of the property has not yet been carried out or that the property is not yet fully developed, the dear money. The energy consultant will only take action when he sees money. It may sound strange now, but the house financing has not yet been approved either.

We have achieved Kfw55 with minimal additional effort. Around €500. I believe it was somewhat thicker perimeter insulation.

Otherwise standard.
-Unheated utility basement (except in the stairwell) and two full floors (T9 bricks)
-Triple glazing
-Controlled residential ventilation
-Air-water heat pump

I consider heating in the (utility) basement unnecessary for us. In the workshop and fitness room, it is an advantage if it is a bit cooler; the storage room doesn't mind either. The heating room is always a bit warmer anyway, so I just leave the door to the workshop open.

That all sounds quite good. The option with an unheated cellar somehow also sounds quite nice. It would be my favorite if my wife didn’t have the silly idea of wanting to heat a room (playroom, hobby room with 30m²). Could there be problems with moisture & mold if the cellar is heated unevenly? I am already a bit concerned about that.

We don't even have a controlled residential ventilation system in the basement. Meanwhile, it is completely dry. For the first 3-4 months, I kept the doors to the stairwell open, that was enough.

If the room is not used regularly (as is often the case with basements), I would only set up an electric heater. If you now include the basement area in the heating load calculation, the heating system might be dimensioned larger, which reduces efficiency at low load. In short, if the heating system in the basement is off, the air-water heat pump consumes more compared to an air-water heat pump sized smaller (without the basement).

Interesting approach. Thanks for the food for thought. What are the temperatures like in your basement roughly in summer and winter?