How to build sunken shed foundation with insulation
Change your roof style! It is a shed why have a high pitch? A almost totally flat roof with a membrane surface will do what you want , these hold up well even in costal areas where I live on the other side of the pond, and in some areas even a 1/2 inch two tall ends up in legal issues , a almost flat membrane was the way to do it , it holds up well drains even heavy rainfalls and they can last a long time if properly installed, want to go a bit cheaper rolled roofing that is heat sealed , not as good as membrane but much cheaper. I agree icf are blocks for walls not foundations, high density foam under a slab is a cost effective way to insulate and add a hydronic heating system and it could be quite nice.