Thank you Perlenmann,
I think I will still hear/read the advice about getting help from others quite often. But I still want to try it myself.
I followed your tip and removed the corners/angles. At first, I didn't want to have the wardrobe under the stairs, but as you can see, it looks good and the WC and utility/laundry room are immediately larger. The breakthrough will be a glass sliding door, meant to convey an open character but keep odors and noise in the kitchen.
Regarding the bathroom in the granny flat: there is no shower, but a bathtub and a washing machine. At the moment, we have a WC of the same size and it works quite well.