How to raise the ceilings in a kitchen ...

Well, yes...

But it claims the buyers own survey says "floor not reinforced".

The weird thing is

a) The concrete slab looks very thin - 3-4"

b) If there is such a void under the floor, even a 4" reinforced slab wouldn't have stayed there long. I'm not sure how a 4' deep void could have "suddenly" appreared, so I assume the floor was designed to span it.

So I'm wondering what we cannot see in the pictures - was there a concrete beam or 3 that failed? Some brick pillars? It all looks very odd as the only type of concrete floor that could withstand spanning a void of that size that I've personally come across would be a beam and block - and they don't usually fail - at least not all at once.

Which makes me suspect the original design was badly bodged - but I still cannot see how exactly.

Water leaks can cause voids like that.