Thank you all - all very informative, even with the basic info that I have provided, and aids my conversation with SE and contractors.
To answer a few more questions.
1. Pool went in after side extension sun-room - foundation depth yet to be determined (built in 1977 brick/block and has roof terrace so hopefully solid foundation)
2. Pool has concrete base/structure with sloping tiled liners. Max depth 1.8m
3. Pool drainage pipes go in perpendicular lines and dont fall under the proposed build area
This view hopefully is easier to follow in terms of what I want to do i.e use the rectangular space as a living area.
thanks everyone again !