You have to apportion the gain according to the time you rented the room and the proportion of the home rented. So if you had a gain of, say $50k, and you sold after owning it for 2 years, I think it would be something like capital gain $50k x 6/24 x 0.2 (20% of the house) = $2,500. Then there is the 50% CGT discount, so $1,250 to pay. Does that sound right?