If you need to put "rules" in place, and your child doesn't know how to behave, then something went wrong in the preceding years.
you are talking about the same thing, you are just choosing not to use the word "rules" 
it doesn't matter if it's common sense to you - and your kids - and normal things, they are still "rules".
I disagree with you OP, it might be the kids home, but it's still YOUR house, they are free to get their own - and should eventually. It's the same thing with a tenant, they might make the property their "home", but they can't make any real decision about it.
Your kids don't become your equals as soon as they reach 18, you are still the parent - as such you should still support them, help them out as much as possible. If you can afford it, you should still support them financially, I never understand these posters who refuse on principle to pay a deposit for a house or for their kids weddings. If you have the funds and the kids are not lazy bums, why wouldn't you help them? You chose to have them, you are still responsible for them.