That sounds fairly soft. If you can push a finger in 25mm then it's going to be an engineer designed foundation, if only 10mm then there's hope yet! However, unless the trench sides have been shored up properly DO NOT physically get in the trench for your own safety. The engineer should do a test as I mentioned - it's called a shear vane test - and the engineer can advise you from there what foundation design can be adopted. Raft foundations are good in soft soil because they effectively float on the surface of the ground and are relatively cheap compare to other options but you'll probably have to fill in what you've already dug out.
The engineer would also have to design the timber frame structure for you as wind pressure needs to be accounted for. Single storey timber frame these days is normally a single skin of 140x38mm studs with ply sheathing and full fill mineral wool insulation (you have to get a U-value of 0.28 so check this is enough with the insulation manufacturer) and your choice of cladding on the outside. Brick facing is more difficult than a render or timber boarding.
The engineer would also have to design the timber frame structure for you as wind pressure needs to be accounted for. Single storey timber frame these days is normally a single skin of 140x38mm studs with ply sheathing and full fill mineral wool insulation (you have to get a U-value of 0.28 so check this is enough with the insulation manufacturer) and your choice of cladding on the outside. Brick facing is more difficult than a render or timber boarding.
