Try - David Horton at DLH Construction, T 415 595 0488, E: DLH07@comcast.net, based in San Rafael.
Do you have plans / drawings and an idea of what you are going to do, do you have the lot size & will the city allow this expansion? So he can give you an accurate estimate range.
Nigel Faulkner, Realtor
Frank Howard Allen
T 415 298 2485
Typically you will pay $400 per sq feet in this area. You might be able to get a little cheaper due to the economy but this is the typical range for Mill Valley, Corte Madera.