In the UK, what is the classification scheme for outbuildings such as sheds and garages?
You'll need to check your local codes. In the US, most building code sections begin with a list of definitions, and I assume it's the same elsewhere. If the difference between a shed and a garage is legally significant then they probably define it. It could be based on size, intended use, access to utilities, proximity to other buildings, access to a driveway, construction technique... really anything your local building regulator has decided.

If this is a structure that might need a permit or inspection, you could also just call the relevant office and ask them.