18yr old son that works and stays in our granny flat, he contributes zero, eats as he pleases gets his washing done. The problem is if we go against him he gets violent with myself and my husband even his sister. He smokes dagga and if I tell him to leave or contribute, he threatens to burn the house down. HELP!

Call the cops to escort him out. He obviously is a danger to your family when upset and is doing nothing to contribute. Don't tell him your calling the cops... just call them. Then when they get there tell them you need help getting your son out.

The cops won't escort him out in most places, because he is a tenant now whether he gives you a dime or not. You have to evict him according to the laws of your state. This starts with a notice in writing to vacate, followed by a court appearance, ending with a sheriff removing him.
IF he threatens you, that IS a matter for the police. Then you get a restraining order.
It's a delicate matter handling this in a way that works quickly.
I would go down to the police station and discuss the threats and a R/O, because many police stations operate differently regarding family who threaten.

Violence and threats are best met with calling a cop. A danger can be removed immediately and you may have to press charges.

I think that you must already know that you have the option of calling the police when anybody threatens you, harms you, or says they are going to burn down your house, whether it's your son or not.

And I doubt that his behavior is something new, but rather part of a long history with him doing as he pleases without consequence, yet still getting his laundry done, as well as enjoying free room and board.

My question is, why haven't you done something already. What is stopping you?