If a garage is a single skin of brick does this mean that to add another storey on top we have to increase the foundations and then double skin the wall?

You would have to dig down in 2 or 3 places, to find the existing foundations, and the depth of them. Its very unlikely that unless they are 1m deep you wont be able to build on top. If the foundations are ok, and bco are satisfied, then you would need a structural engineer to design steels, and maybe extra piers. Whichever route you take, you will need a S/E planning and building regs.

We have found it is cheaper with all the steels, concrete pad foundations, messing around etc to just pull the garage down and start again. Unless it is a very large one .