In which case as already mentioned, Party Wall Act will apply, as will planning permission. Both of which you will be able to oppose, but no guarentee of success.
I suggest you tell them that you'd most likely oppose any such development and ask if they can avoid that design. However, you might have to concede as your extension has got there first and plonked itself on the boundary, which leaves them little option but to build onto it, just as the houses are joined, the extensions will be too.