Would it make sense to place the guest WC where the staircase start is currently located in the plan? The children's bathroom on the upper floor would be directly above it.
I would maybe do it like this... as the parking spaces/garage are now, it is probably not allowed anyway, since one parking space is blocked... garage gone, parking space for the basement apartment next to the basement apartment, which is slightly offset so that the living room can still get a window on the side... lunchtime doodling, template depending on incoming mail