I still have this draft on offer :) I would, however, choose a wooden staircase instead of a concrete one and plan gables at the top instead of a hip roof, so that the house can be lived in later with more options. In addition, a guest room or office next to storage space under the roof would also be nice. Even if your parents apparently want to live conservatively, a sauna or yoga sessions can make many joints flexible again, which do not get better with age ;)