This is a space under the top floor.
One section, about 10m long by 3m at widest point, has been excavated (to a 2.4m ceiling height) and the floor concreted.
Beside that, a 2m wide section has been excavated and concreted, but only to a 1.8m height
At the moment it's full of tools and other vital and essential items. (OK and a lot of junk as well)
Potentially, with very little effort, it can be a huge wine cellar. Or with a lot of effort, you could dig it out, put in RSJ's to support the room above, cut holes in the wall out on to the patio, and turn it into more bedrooms,offices or living spaces