I can very well imagine the living space with plenty of room to grow. Enlarge the bathroom, add a guest toilet* and forego the office closet downstairs. Set up a nice guest area and your office upstairs. Consider planning for air conditioning if you are sensitive to heat. Under the roof in a densely built environment it is especially warm. A large window from the upstairs office into the living space could be nice. Light, contact, cross ventilation...
*Gäste-WC - I like this certainly not necessary luxury. I find it especially pleasant as a guest not to have to intrude into the privacy of my host’s bathroom. As a host, I don’t like having “strange smells” in my bathroom. Whether someone could see that would be quite irrelevant to me but the first two arguments speak strongly in favor of a guest toilet for me.