Welcome Joe,
I am in the bay area as well and would certainly echo Jon Klaus' advice.
If you meant multifamily for-sale (either fee simple or condo) I only see very specific neighborhoods where fee-simple is working (including stacked-flats), but it IS working right now in our area. If no one could convince you otherwise and you are hell-bent on jumping into a ground-up project, I would focus on this product type but again, only very specific neighborhoods, and only with an experienced contractor.
If you meant apartments, I would just point out that in our area rents are the highest they have ever been, which you might want to take into account when running the numbers (especially if you are building apartments that can take a long time to build then lease-up).
Commercial can be great, but I haven't seen any non-tenant driven projects in quite some time here. If you have a tenant with good credit ready to build, and you have the ability and connections to hold the paper or sell it (among other things), then that's a special case.
To answer your question about when it makes sense to build new with regard to multifamily, I would begin by asking myself the following question: Is anyone else building anything like this right now in this neighborhood that is proving my pro forma. If not, there is probably a reason why - whether its lack of financing, lack of demand, building costs, zoning, whatever. Maybe you have the skill, connections, knowledge or talent that allows you to overcome all of these things - people have done it before and have been very successful. But consider the possibility that at this stage, you do not. I would just caution you not to be a pioneer on your first project - that's how it can easily become your last.
The best advice I ever got (and often repeated on this board and elsewhere) was to find a partner or mentor with proven ability to teach you the ropes. It is certainly worth repeating again.
Hope this helps,
Ben