I guess that if it is joint ownership, then you would both have to agree to what is there so its best to befriend them and try to get them to buy into your idea.
More importantly, there is usually only one winner in these kinds of disputes, and thats the solicitors, please do all you can to avoid getting in a situation where you need to employ one.
Maybe get a party wall surveyor involved, but let your neighbours know first as that will antagonise them if you don't.
Good luck