I guess this goes for all footings when you are building dependant on climate, locale; I would say where I live is pretty temperate so my footings might be only 1 foot Ask your local council when asking building permission for best advice.
Big plans for the future. Get More Garage Storage With a Bump-Out Addition- This would be awesome. It could 'bump out' to the garden area. Decorative windows would be cute and the garden arrangement would compliment the whole thing nicely.