Extension or knee wall height increase?

I wonder what you gain downstairs, that is on the ground floor, with the extension. The sofa should also fit in like this now, the dining table as well. I do not see an extension as drawn as a gain. The shape is rather too long for a living room. It is more that there is a lack in the kitchen area. Personally, I would rather enlarge the more active area. However, if it is primarily about the upper floor, then this should be included here. Is an extension possible from a building law perspective?