Ideas for improvement

Two-storey is allowed ..

That is not compatible with the architect's idea of extending a bungalow or designing the possible buildable area as a split level.
However, it is better this way, as the basement can be fully utilized and used as a foundation.
Long story short: you are not allowed to build another pitched roof.
But you are allowed to convert an attic.
So if you find a place for a staircase, you could either have a steeper roof pitch or a knee wall so that living space is created under the roof.
The former Okal house roof pitch actually fits the house type quite well, or rather the house it was meant to be.
Therefore, for example, you could 1. only raise the upper roof half so that one or two rooms can be set up there. A third gable would be created in the middle of the house, i.e. an offset between the roof surfaces, which many equip with bands of windows.
Or you take 2. the lower split level and try to create another living level above it by having the whole roof at the same height so that space arises above the living area.
For both options, you could deduct one meter from the children's room at the hallway and build a staircase up there. The then smaller children's room can later become a guest room/office, and the child moves into the attic level.

Hello, yes that was also a consideration - possibly also creating access through the dressing room, so that a space can possibly be created above the bedroom .. I still can’t imagine it

Are there any suggestions?

I would probably plan to completely renovate the roof. Knee wall around 1.20m and then make sure it does not become a full floor. Child on one side, additional room on the other side. However, everything downstairs would also need to be rearranged.

Convert the roof above the bedroom into a guest-snoring cabin and use a space-saving staircase or ladder, e.g., from the hallway with some bathroom space starting at 1.5m height. Ceiling in the children's room up to the roof creates space for a loft bed as soon as falling out is not an issue. The architect's design looks comfortable to me.

Are there any suggestions?

Have you ever calculated completely demolishing the old house and building a new house with a ground floor and attic?
You invest a lot of money for an extension and new structure, but still only have a compromise solution.