We have a breakthrough

 
The bifold doors will go here. The external ground will be lowered by about 600mm and the new insulated floor slab will be about a foot higher than the sand. A nice project. 

On another matter, I was able to help the builder (we help each other really) to resolve issues for the drainage, structure and the spatial arrangement.  I attend site partly because I am a nosey person, but also because my role as the architect is monitor the progress of the works and assist with the technical design. So I am there assure the quality of the work. I like that too.