1.5m x 2.5m single concrete block shed 2.5m height

You should have structural engineer drawing for that but usually 80 cm should be enough however you need to reach clay as well and on the top of the clay you can build your foundations

I would install a concrete pad 175mm deep with 150 mm compacted stone install steel mesh and poker the concrete pad this will be ample for a small shed

All depends on the soil conditions and if there is any trees near by, Thus either a competent architect or structural engineer should provide correct details of foundations, this is due to this building being a solid build and not a timber frame. also does the sheds location meet planning guild lines.

Hi. Firstly consider the following. -Does the proposed development comply with local council regs. -You are not building over a shared or main sewer. -Are substrate conditions appropriate. Strictly speaking you shouldn't need a calculation or drawing done. It would be prudent to simply call your local council and check. My advise would be to lay a "stepped ground bearing slab" This is effectively a concrete slab with slightly deeper perimeter around all sides. You will need to excavate enough to accommodate for some "typ one" hard-core which should be mecanicly compacted prior to concrete pour. A sheet of weld mesh (reinforcing) could be incorporated also. This may at first glance appear to be an excessive option but you will have less spoil and the floor is done at the same time, ready for block laying. If you have access for a concrete truck or evan a truck and a concrete pump then this undertaking becomes very easy. You can hire "concrete formers" most places to make up your shuttering. Regarding the hight of finished base, try to ensure that the top of the base is just above surounding ground level, this way you shan't have excessive damp. Hope this is helpful. Regards Louis Kingwill