In my opinion (SF general contractor), no, $140/square foot for an addition including master suite, whether or not it includes fees for an architect, is not doable in Noe Valley in 2012. Rock-bottom would more likely be around $250/square foot, and typical would probably be $350, luxury much higher. If you get a contractor who promises you the job for $140/square foot, get a firm contract then spend a few bucks to have a third party (a contractor who isn't bidding for the job, or a broker with development experience) go over the contract.
You're doing the right thing by talking about budget up front. It's a starting point, from which you can talk about changing your budget or the scope.