need rough sf numbers on additions/new builds

[Reggie Burnett]

Working on a deal on a very small house (about 900sf) on a 1 acre lot.   I think it needs to be bigger. The house is not in great shape so bulldozing and starting over is an option.   My question is what are some good per square foot numbers for adding on to a house vs building new if I'm the GC on the project?  I've heard int he past that houses with average build out quality can be built for 60-70 per SF.   Is that too low?  What about adding on about 300-400SF?