Adding square footage to an existing home - fix and flip

[Tasia Turner]

I am looking at a possible property to purchase and adding about 400-500 sf of extra living space (den/family room) would really increase the comps for this particular home. I am trying to figure out a ballpark number for adding on an extension. Does anyone have a good tool for evaluating this? Or a good cost/sf to use as a basis? I am located in Northern California and the house is in a rural area in El Dorado county. Thanks in advance. I look forward to hearing everyone's  thoughts on this :-)