Recently purchased a country property where the previous owner poured a solid 30x40 ft foundation, apparently for a shop or garage of some kind and never completed it.

This foundation is completely solid with no plumbing coming through it. We would like to use this foundation for a small cabin or metal house since this is a really nice, thick pad.

Question : In order to add plumbing, could I just add 6-8 feet of extra fresh slab at the rear of the present foundation with the plumbing in place, and have the kitchen, bathroom and laudry located near the "new" slab addition?
Someone mentioned to me of cutting through the slab to add piping, but that seems like a lot of work and maybe damaging the pad.
Another option would be to have it completely demolished and starting fresh.
Any suggestions appreciated.