Advise please, our foundation is extending to neighbours property

[mrrusty]

They are allowed to cut away your foundations BUT - BIG But - they can't allow your foundations to become unstable "There is a legal right under The Party Wall etc Act 1996 for the Building Owner to trim and cut away the projecting part of the neighbour’s foundation, but must ensure it is not made unstable in doing so."

more here https://arunassociates.co.uk/building-adjacent-to-neighbouring-property/

 

[noseall]

Don't know why they don't just butt against the existing and build an eccentric footing. It's unlikely to rotate as its butting against another lump of concrete.