https://www.loveskirting.co.uk/flooring-thresholds-c17/by-product-c29/ramp-section-c43/solid-oak-ramp-section-threshold-0-9-metre-p121

Ask your self if the kitchen is actually a good kitchen….. if you need a utility room next to the kitchen for additional refrigeration or storage or additional work surface then it’s not a good enough kitchen for you. Ideally the utility should be just a laundry room in which case it’s best near the bedrooms upstairs or at least near the bottom of the stairs.

If you made outer wall then its okay to build window on the side, your neighbor privacy will be maintained and you can get the sunlight needed, I would advise to move the garage to the front so it can be accessible for a car from the main road, you can still have direct path to the garden from the front if you move the enterance to the right and just put a fence, if it were me here's the layout i will do, it'll give more space for closet, bigger bath, utility room, and additional storage unit, so basically move the wall to the left and focus buildng that side, the previous garage will become the new master bedroom with en-suite

I'd also be inclined to swap them as Jonathan suggested. Check fire regulations, as perhaps you do need to rotate the stairs to allow an open kitchen. (I'm in the Netherlands so can't advise on these.)