Production companies and TV execs want 'originality' yet how to write comedy books insist on characters in tried and tested settings such as families, flatmates. What to do for the best?
Sitcom - setting or character.
The trick with a sitcom is that you need a variety of characters and lots of conflict between these characters BUT most importantly you also need these characters to spend time together which is why you need a trap. So that these conflicting characters don't just say "I don't like you any more, I'm leaving this place." And that's the end of your sitcom. So the most obvious traps are family and a flat share.
But of course there are all sorts of families (Browns, Khans, Trotters, Griffins) and flat shares (or places people are forced to be together) such as The Office, Blackadder, Fawlty Towers, Friends, IT Crowd, Bad Education, Not Going Out - pretty much most sitcoms.
This is a very simple explanation there are more layers to it.
The originality is going to come from the tone and style more so than anything else, your setting doesn't matter too much - except, as ContainsNuts says, it has to trap them in some way.
Your characters don't really have to be massively original either, I don't think. I.e. all sitcom characters tend to be the same - losers with ambition - and that works. So you don't need to necessarily be thinking 'what hasn't been done before', you just need to make sure the characters feel real and normally that they want something they will never get - though obviously that's where the rules can be bent.
I tend to focus more on voice when it comes to originality - i.e. not mimicking someone else's voice but cultivating your own.