Digging out cellar floor to get a bit more head room

[codan]

I have a cellar in a 150 year old stone built house. The floor is cobbled. I want to dig out the cobbles and a bit of substrate so I can get another 6 inches of head room.
I have a dug a test hole by one of the walls, the cobbles are pretty loose and easy to remove, under the cobbles seems to be just compacted earth. Digging by the wall there seems to be no solid foundation under the wall. Is it likely that a 150 year old 2 storey house would have no foundation or have I just found a dodgy spot?
Do you think digging out the cobbles and laying a concrete slab will weaken the structure?
The room is about 3 x3m. I plan to put in a membrane and a 2" reinforced slab. Will that be OK?
Thanks

[welsh brickie]

seek proffesional advice.
You could be undermining the entire structure.Some houses where built
with no foundations only using bricks laid on a header course.

[codan]

Thanks.
I did ask a couple of builders and the advice was to underpin the walls. I am waiting (with dread) for the quote.
Cheers