Another thing you could also consider would be to use lighting in different areas of your garden which would also give you some nice lighting in your patio area you have planned for some alfresco dining maybe? These are ideal as they let you concentrate the light where you want it most, allowing you to have a stunning designer look to your garden. Love the amount of space you have though so you will have to post some more pics on here once you have the work done as I'm sure you will get some great suggestions on here as to what to do.

Are you sure you def want to level up the end of the garden? I think it could feel quite exposed there as your fences are lower at that end. How about digging into the slop to terrace it instead? I think in terms of budget, turfing and decking would prob use a good proportion of the amount you've mentioned. A cabin would be lovely but would def require more budget to get something decent.

Looks like a good quality job. I like the steps and the garden office. It does feel rather 'linear' to me though. All straight lines and angles, which seem to produce a somewhat harsh look, not helped by the formal style planting. However, if the client is happy, It's a successful make-over.

I agree with you that if the extension is 2.7m deep internally it isn’t big enough for a living or dining space so keep it all one level. You might want to investigate the possibility of putting a spare bedroom or playroom on a semi basement level below your extension as it may not be significantly more expensive than what you will have to build for your planned extension. You might also consider the possibility of building an alternative extension that is not full width but is deeper if that makes planning the space easier