I have spent too long looking at this and have decision paralysis. I am sure there is a good/creative way of doing this so would love some fresh eyes.
Standard rectangle shaped side return extension at the back of the house.
Layout as below. Ideally I would like a kitchen, dining and sitting area but understand it is difficult to fit all of that so prepared to compromise. Sitting area is not essential but somewhere to eat dinner is.
The room has water/drains on both sides - shown in blue.