Beyond light/shade, are there any practical reasons for the 'rooms' to go in a particular order?
Bins are better kept within a reasonable distance of the back door - you don't want to be having to traipse a long way on a dark wet winter's night to put the rubbish out. And preferably don't have a gravel path as it makes it difficult to wheel the bins out.
A veg patch needs to have access to a water supply, and think about electricity supply to outdoor buildings too - it pays to install that early on in the planning. If you have grass at the end of the garden and an electric lawnmower, you will thank me for this 
A sunny spot near the kitchen door is a good place for a herb garden, otherwise put it near a seating area, as herbs smell lovely in the sunshine. They do quite well in pots anyway.
If there is to be a sandpit then think about how to cover it when it isn't in use, otherwise the local cats might use it as a litter tray.
Don't site a barbecue so close to the house that all the smoke comes in the windows (either yours or your neighbour's), but don't have it so far down the garden that it's a nuisance to carry all the food down there. It's best not to have it under overhanging tree branches either.
The washing line is best sited near a paved area and within a reasonable laundry-carrying distance from the house.
Think about storage for things like bikes, outdoor toys, garden tools, watering cans etc.
Basically, make a list of all the things you need, the things you'd like to have, and then compare all that with what you've got now.
No pressure 