How deep will the trench need to be for 20ft room?

I would recommend going to building regs because of the size of the outbuilding which is 1 metre deep by 2ft wide, until you find hard ground is what building regs specify. You can mass fill with concrete or put minimum of 150mm concrete and build the rest up with blocks and bricks!

In Scotland a standard foundation for a simple double leaf and cavity is 600mm wide if using double skinned 100mm block with a clear 50mm cavity , or 700mm wide if using an internal block of 140mm to accomodate a 140mm kit build. A minimum scarcement area ( the overlapping concrete to the outside and inside not built upon must be a minimum of 150mm on each side ) The depth is normally 600mm minimum with 200mm of concrete poured at this level leaving a depth of 400mm to build up to reach ground level. If after digging 600mm you have not found stable ground you can with the permission of the engineer put in a type 1 base and after compressing it should be suitable. Very rarely if ever have I dug lower than 700mm. Hope this helps.