I think having a toilet in the garage is great. This way, the kids don’t always have to go into the house and take off their shoes when they need to go, and the parents don’t have to either when they’re working outside. I have wished for a garden toilet more than once. What kind of heating is planned? I also find it very unfortunate that the most beautiful rooms upstairs have to serve as the heating room. We have a bathroom in each of our two dormers. If you still want to keep it this way, I would let the heating room extend across the entire width of the gable and maybe put the washing machine and dryer in there as well. This way, you save yourself the hassle of carrying laundry up and down. I would place the chimney flue more in the hallway than in the kitchen. Upstairs it would then be in the corridor, and that would probably work. I also wouldn't like the shower opposite the door in the bathroom. I find it impractical if someone is coming out of the shower while another person wants to go in or out.