Is the utility room sufficient as a storage room as well?

A fixed staircase requires a small hallway at the top and is only practical with a 30-degree incline, otherwise there is not enough headroom at the top. An unheated utility room is quite cool despite the installed equipment.

I don't have a house yet, but I have a 100 sqm apartment and a 20 sqm cellar plus a laundry cellar (which is only used by us). There are two of us, and we manage to fill the apartment quite well; the cellar is definitely FULL as well. We have a lot of camping gear and like to tinker (we have a workbench in the "study"). The laundry is still hung in the future children's room, later then in the cellar or outside (ground floor apartment). If there is ever a house, it will probably be without a cellar, so it should have a large kitchen where everything fits, a technical room where only technology should go, a dressing room where winter and carnival clothes also fit, a utility room on the upper floor where all the laundry can be done and where cleaning stuff for the upper floor also fits, etc. In my opinion, even without hobbies you can't have enough storage space. And my husband and I really do not tend to keep everything.

In our building services room with air-water heat pump and [Kontrollierte-Wohnraumlüftung] it does not get warm at all. The heat pump is completely indoors, but it should be the same with split units.

In addition to the utility room, which at nearly 10 sqm is not too small and will house a washing machine + dryer, we also have a storage room of 5 sqm for beverage supplies + miscellaneous items, as well as space for the vacuum cleaner & co. behind a closed door under the closed staircase, as nicely shown here under #3.

and we are wondering if we might possibly have a flaw in our thinking somewhere and therefore wanted to ask around how you see it.

I wouldn’t call it a "flaw in thinking" that not all building families are the same. In the "worst" case, you have one more room than Müllermeierschulze. House manufacturers are used to tougher situations than that. Just google with the additional search term "customer house": the takeover of the catalog design 1:1 is rarer than some think.

Maybe it simply depends on what one is used to. If someone has lived in a rental, they might manage with less. For example, I would want a small storage or pantry on every floor. A technical/heating room. A room for washing and hanging laundry, possibly in the heating room if it's large enough. And a storage room by the garage for tools, bicycles, lawnmower, etc. That would be my minimum.