How to obtain an exemption from the development plan?

You must not expect too much. This is not a cabinet or anything similar. To put something in there, you almost have to lie on the floor. You should keep that in mind.

Katja, that is clear to me. You also have to see how far you want to take it. But I imagine, for example, putting the junk at 1.20 and then a compartment at the back, either only about 1m deep or all the way to the end. Since the children's rooms are going in up there anyway, a large part would be storage space for all sorts of children's stuff. And a work/storage room. You can use the storage for things you really rarely need. For example, Christmas decorations. Soap-making utensils, spare bulbs, etc. I will discuss this with my wife. But I think it should basically be practical.

Our knee wall chamber has a "clear" maximum height of 1.20 and then slopes down at a roof angle of 27°. I am quite tall and can crawl inside, going in forwards and mostly coming out backwards. Since the things inside are organized in boxes, I use a kind of Tetris sliding method to get to what I want. Old financial folders and stuff like that are all the way at the back. I am 50 now. In 20 years it will be difficult for me, but by then the children's apartments will also have become free. I "crawl into" the knee wall at most once a month.
By the way, we have two such knee wall chambers; the other one is accessible by ladder and is above the bathroom and is visited very rarely. In the children's apartments, we are also building one each.
I find it space-efficient, my wife does too, but I would never crawl inside myself (claustrophobia). Good hiding place for gifts.

The idea was already mentioned. I didn’t know it was exotic for you, since there was no demand

Normally on the exterior wall and roof, so you can optionally set your knee wall and use storage space behind the knee wall.

That way every child’s room could have its own small storage room. And the actual storage/work room becomes a bit bigger. Then it just gets a partition wall or something.

It looks more or less like that, whether with doors or without...
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You can also take Kallax, turn it, put wheels underneath and attach handles on the sides – lots of things possible

Yes, you are right, there are already quite a few possibilities. What you must not overlook, however, is that we are allowed a max. 28° roof pitch. That is already very flat. So not comparable to the photos you posted. As you can see, with a house width of 10m, I have barely 4.5m where the roof slope is over 2m. This is not meant to be "complaining" again, but just to explain why we would find 50 cm more knee wall already quite nice.

Would it perhaps be more acceptable to build a non-full story (the upper floor steps back at the eaves sides) and then put the gable roof on it? Then you wouldn't have to convert the "dead" space of the storage area at all.

NB: That would definitely be different as well.