This is a difficult question to answer, and I'm not so sure any answer would be correct given the personal taste of the actual homeowner (think formica vs. granite as an example). Location of the property can also come into play; there's a big difference building on a flat lot vs. a home on the side of a hill that requires 30+ feet concrete piers.
In any case, the link below may be useful:
I will attempt to answer your question and inform others too.
Clearly, you need to know the market demand for contractors at this time. There are quite a few builders right know looking for work and this means prices are soft-ish. Read the answers below for more insight but I would tell you people need to know specifically what work you want and then get bids from three or more trusted contractors.
It used to be you could say $500. or $600./ square foot ... not true as much today. Get a plan. Get a bid.
If you or anyone out there needs a trusted contractor in Marin or San Francisco contact me. 415.497.8864
Be very careful in accepting any estimate of average cost to build. There are a LOT of variables, but even more importantly, you need to know what is INCLUDED in the estimate you are being given.
Is it just "hard" cost (the materials and actual labor to complete) ? Or does it include some of all of the "soft" costs (builder overhead and profit, insurance, financing fees and interest, design review and permit fees including possibly school or other community fees, building permit fees, geotechnical and civil engineering costs, architectural cost, utility fees and installation,bonds or offsite improvement costs of city or county). These can add a substantial percentage to the "hard" costs and in some cases may be more than the hard costs. The size of the project is also important. A smaller project may have a higher cost per sq ft than a larger project. The access may affect costs as will the location of necessary utilities and cost to bring them to the building site.
Your can buy a custom designed prefab house FOB (at the factory) for under $100-250+per sq ft. but that doesn't include transportation, installation or foundation, permits etc.)
Stick building in Marin is typically quoted at $350-750 per sq ft. Hope this help you see how tricky your question is. Maryann
Basic $150 -$250 sf new construction
Basic $300 - $500 additions
All of this is based on what you are doing coupled with quality of finishes.
Ultimately the best answer is get a couple of estimates from a reliable builder/contractor.
But being in Real Estate in Marin you already knew that.