My reading is that there is a path running along the side of the house. Between this path and the boundary wall, there is a strip of grass. OP wants to fill in this grass area with concrete now, but at some point in the future may wish to extend the house over this area.
So question is, is it worthwhile doing this job now and putting in the foundations, DPM, etc. as if he were building the extension, or would it be more cost effective to just put in a regular concrete slab that would have to be dug up again should the OP decide to extend.
My guess is that going down the full foundation and floor slab route is overkill, and may not meen your needs if /when it comes to extending, but there are others here far more qualified than me in this area.
Leo