depth of foundations

[ipmdfm]

we are wanting to dig out our garden as it is on 2 levels, our neighbour is above us and her house is around 1.75 metres above ours. we want to dig out to within 3 metres of her house wall. What would be the minimumfoundation of her house, it is built o a raft, as we understand if we wnat to excavate to more than the depth of her foundations we will have to get a party wall agreement
thanks, I hope that's clear!! :D