Insulation under garage floor

[Auchlossen]

Garage is about to be built into 'hillside', with retaining walls on 3 sides, to varying extents. The retaining walls are to be built with Stepoc concrete blocks with steel reinforced concrete infill. There is an office/studio above. EWI will be used for garage and studio above.

Question is: should I insulate under raft floor with 100mm of insulation? There will be large thermal losses at the edges. Arguably the half buried garage will be kept at a stable if not warm temperature by the surrounding soil.

[Bitpipe]

I put some spare 200mm thick EPS 70 under my garage raft, not expensive and should help keep the garage warm.

[Gimp]

I will be in a similar position with building a garage back into a hillside so will need a retaining wall system, but one that will be wide enough for a cavity wall, 350mm wide to carry on above the retaining wall. I'm not sure stepoc would be wide enough for me though, unless it can be laid double width? 

[Roger440]

I built mine in a "well" of 100mm EPS. Really can tell the difference compared to my old garage which is a slab on the ground!