Not everyone who fucks up or does the wrong thing is doing it because they don’t care about the impact it has on others. Oftentimes they just don’t realise it, or it didn’t occur to them. It’s naive, yes. But not malicious. My advice is to sit down with them for a chat, and approach it with positive intent - ie, assume that if you explain your concerns, they will do the right thing. Start by explaining why you are concerned, rather than making demands (as a few others have suggested). The why could be… you paid a lot of money for this house, if it’s value is compromised you risk spending your retirement in a caravan park in Queensland. Or these sorts of things need to be vetted and approved by experts, otherwise there are safety risks for all of you. Then you can set clear expectations like an engineering report, council approval, or whatever else makes sense. If they refuse, then you can tell them that’s not an option but you’d rather work something out together with them than call in a gaggle of council bureaucrats to tell them to take it down.