Thanks everyone, I agree about making the kitchen/diner very dark....that's my major concern.
A few a reasons I don't want to remove walls to the existing house, money, disruption and I don't have the funds for a new kitchen for a few years (am mortgage maxed out too, so that's not an option either).
In answer to a few questions:
There's loads of garden left, the studio replaces an existing old garage so I don't lose much space. There is still loads of room for lawn/planting/patio etc.
There will be a back gate for access once the side return is filled in.
The house has 3 double bedrooms upstairs, fully occupied, only one bathroom upstairs.
The kitchen is a bit small, it's a 3 bed semi, but I don't like huge sprawling kitchens, I don't spend much time in the kitchen and never would - unless I got an aga - then I would probably sleep there!
I'm not bothered about the utility room being separate and a faff to get to, it's a compromise to do with my points above.
My fit bit will thank me for all the extra steps although at the moment I trudge down the the bottom of the garden to my washing machine in the garage, so this will be better than that.
I have a dog and a cat who both use a cat flap, so that needs to go in the design somewhere too.
But in mind with everyone's comments would the plan below be better in terms of light?
I can create a little tiled courtyard/sitting area, it would mean more light?
Plus I wouldn't have to move the drain which would save a bit of £££
My main objectives are: more space (obvs), as much light as poss & good views of my garden which is lovely and green and natural.