Let me explain the basics:
Some houses, especially terraced are built on what builders call "slabs".
A few years ago, instead of digging footings where the supporting walls would sit and later think about flooring, they used to pour concrete over the all area of the terrace and create a giant slab.
Then they would build on it having to bother only about screed internally as they already had the base or joists and floorboards for suspended floors.
I'm not a structural engineer but I guess that having this large surface would spread the weight and so it could be thinner than today's footings.
However, like me, a lot of us here have seen cracked properties because the slab sometimes doesn't take the weight.
It's a free lesson in basic (ancient?) construction.