Obviously it would be easier to buy the 'kit' as the plan is already done for you, making it yourself would give you the made to measure size if you're capable of making one, I have made 3 sections summerhouse in a L-shape format in the corner of my garden and have saved a lot of money, like I say depends what your capability of making one, there're some plans on the website to help you to make shed/summerhouse.
Do you need planning permission for that? Was trying to find the website of a company I know (Nordic something) who make timberframed garden offices, but can't find the documentation they once gave me when I met them at an B2B exhibition (filed away to well )
Isn't a garden office regarded the same as a garden 'shed'?
Normally you just need to follow building regs unless it's over 30m, otherwise it will need planning permission. As for office building, I'm not up to date now but I think it's to do with running a business from home and depending what the office is for etc