My first Sims 3 home I created from scratch just a few days ago.
I haven't had time to go back and play it again since Jun has been hogging the pc like mad and the only chance I get to play on my own frikkin pc is when he's asleep and by that time, I'm too sleepy myself to get up and play for hours -.-
I always start with the interior and work around it. So I would place where I would want my furniture to be and then place walls around. But what I find is, I end up with one whole wall with none or not enough windows. It just looks silly. I was going to frap this, but it takes a while for me to create a house since I'm picky with colors.
I don't really (or haven't) made a mansion. I like to create cosy little homes so that it doesn't take half a day for my sims to get to the bathroom.