Hi John
I built my workshop in the garden. It is 260ft², wooden construction, flat roof.
If I owned the whole house I would not have needed permission, but as I live in a flat, I did. The restrictions here in Sussex are maximum heights for flat and pitched roof structures, and the total area of the structure must be within a specific percentage of the whole area of the entire plot that you own.
This percentage was high - I could have built a workshop over my entire garden! There were no restrictions regarding proximity to the boundry walls or anything like that.
My local planning office were very helpful at the beginning stages of working out what I could have. I remember being very surprised at how much I was allowed to build without permission if I owned the whole house!
Regs will no doubt be different where you are, but I'd recommend putting together an idea of what you think you'd like, making up some simple drawings of the entire plot and then making an appointment with a planning officer to discuss the options, whether or not you'll need permission and what restrictions apply to you.