@Chunkymenrock no definitely not mine. Never even been to Twickenham 
@AtillatheHun you're right. I hadn't thought of that! Definitely 2 lots of council tax
And as others have pointed out, presumably the advantage of having a separate self contained area for e.g. AirBnB, lodger, adult child wouldn't really work as they'd have to traipse through your home to get to theirs.
I'm going to keep an eye on it to see how long it takes to sell and what it eventually goes for..