Floor plan of a multi-family house with 4 residential units

The staircase has 17 steps and a rise of 17,xx, which in my opinion is quite a waste of space. I would consider lowering the ceiling height a bit to save one step and corresponding space. I would partition off this storage room and part of the hallway just behind the children's room door and add it to the living room as a kitchenette. If the stairwell can be shorter, this corner will also become larger. In the bathroom, I would also omit the bathtub and instead provide a space for the washing machine.

I would section off this storage room and part of the hallway just behind the children's room door and add it to the living room as a kitchenette.

I don't think that's a good idea. A storage room is always useful; items like vacuum cleaners, mops, smaller supplies, cleaning buckets, and all that kind of stuff have a good place to be stored there. And for a kitchenette, the partition would somehow be quite unfavorable. It creates useless corners for the kitchen. Stair 17x17.5 results in a floor-to-floor height of 297.5 cm. With current assumptions for floor and ceiling structures (possibly integrating ventilation), this results in about 255 cm of clear room height. The riser height of 17.5 cm is also still comfortable, even if one has knee problems, for example, in old age. It is the main access to the other apartments. The basement is supposed to accommodate the washing machine. The washing machine could also fit in the storage room if one wants to have it in the apartment. The intended storage room corresponds quite exactly to the dimensions of my storage room, and there I have all the above-mentioned items as well as the washing machine.

The bathtub belongs in the bathroom. kbt09 already has a good solution for K/E/W. The original plan was nothing. Somehow, light should still get into the hallway --> darkroom. Is a door to K/E/W even necessary? --> glass wall without door

I don't think that's a good idea. A storage room is always useful; things like vacuum cleaners, mops, smaller supplies, cleaning buckets, and all that stuff can be well accommodated in it. And for a kitchen corner, the partition would be rather inconvenient. It creates useless corners for the kitchen.

I do agree with that, but I find the living space otherwise very cramped. An open space with only 31 sqm – that's quite tiny, isn't it?

The riser height of 17.5 cm is still comfortable, especially if one has knee problems etc., for example, in old age. It is the main access to the other apartments. The basement is supposed to be used for the washing machine.

That's sweet. You want to chase the elderly to the first floor and have them regularly do their laundry in the basement. That keeps you young! ;) I don't know why they plan a ceiling thickness of almost 45 cm here. Does soundproofing play a role? Otherwise, I would find that exaggerated.

Living area with only 31 sqm - that’s quite small, isn’t it?

We are talking about a 3-room apartment with ~70 sqm here. You can’t make big leaps and almost half of it being the living area is already okay. The target group is probably a couple with one child or a childless couple with one person working from home, so the size is sufficient. Of course, generous is something else, but you can’t apply the standards of a single-family house twice the size here.
Such a storage niche (maybe leave out the door and just use a curtain?) is also practical, because there aren’t many storage options. That’s why I also advocate leaving out the bathtub and instead putting a few cabinets in the bathroom. I now see little sense in enlarging the kitchen into the corner.

I would keep the door between the living area and the hallway; you might not want to hear everything happening in the stairwell. Possibly a transom above the door or just simply turning on the light in the hallway.

Regarding the true ceiling height versus the floor-to-floor height, there is on one hand the pure ceiling between the floors and then the floor construction with insulation, underfloor heating, screed, covering, etc.

And these will be rental apartments, and depending on that, as a potential tenant, you don’t have the choice to rent your optimum living space, such as on the ground floor. Or, you simply live there longer and slowly grow old.

With the all-purpose room K/E/W, I see it like ... Such a storage room is worth its weight in gold. In my previous apartment, I didn’t have one; then the vacuum cleaner is in the niche behind the bedroom door, the mop bucket ... well, where? That was a problem, had to go into the kitchen cabinet, the mop into a niche in the kitchen, which at that time wasn’t planned wall to wall, etc.

I would definitely choose a door for the all-purpose room K/E/W, it can be a glass door.