Summerhouse footings

Evening,
I'm digging the foundations for a 5.5m x 5m single storey summerhouse constructed in 4x2, osb, then redwood cladding with a pitched slated roof. I was going to make the strip footings 600mm wide x 600 ish deep (depending on the quality of the sub soil), but looking at it 2ft wide seems over the top for this building, do you reckon I could make the strip 300mm wide as the overall wall thickness will be approx. 150mm?
Thanks.

Hi, CCW,
for a light timber building like yours, and depending a bit on the ground.
I woudn't go much over a spade depth, (9") and dig it out all over the area , put some harcore down in the middle up to near the edges,(but leave the edge 9" deep so there is some 'meat' where the load is) about 4" down from path level and concrete the whole area as a slab.
you can always pave on top if you want it more decorative later, in which case make the concrete 3" lower than Finish path.