Hi Gill,
If you are building from scratch then there are several options. The first is a concrete slab. This is solid, will never move and support anything you want to put on it. The down side is they are a pain to lay, cost alot and are cold to work on as well as the obvious problem of dropping tools onto!
A cheaper and slightly easier option is to lay slabs (900mm by 300mm by 50mm thick) just basic grey paving slabs on a hardcore and sand base. These still have the draw back of concrete in terms of coldness and damage to tools, and can wobble if not laid correctly.
I would not lay wood directly onto earth, even treated timber. You could lay timber onto concrete pilings, either slabs in each corner, or I have also seem concrete poured into drainage pipes as supports.
The way I did it, and I assume most others do, is to lay a concrete slab, then build a wooden workshop that sits on it, laying a wooden floor inside the walls of the workshop. I have a concrete slab, then a layer of damp proof membrane (big single sheet of blue plastic from builders merchants) then 2" by 4" pressure treated bearers laid with the 4" side down (ie so they are 2" tall) then skinned over the top with 18mm OSB. By laying the floor once the walls are up you can lift the floor in the future if you wish, rather than building the walls on the floor, which makes it impossible to get up!
Best of luck, seems a bit cold to go digging up the garden now though - wait till spring arrives and the permafrost has gone!
Steve.
