Rik - first of all, you live in the Bay Area, so a greenhouse is not really necessary (I grew up in SJ). You get a bit of chill wind off the bay, and if you are trying for more tropical plants, a greenhouse might be helpful. In the Bay Area, you can grow almost everything, especially those fruit trees that I can't grow--Elberta peaches and Santa Rosa plums. Also, unless you are in the country, you can grow many plants that get chomped by the deer here. It so rarely freezes, that a "walapini" isn't needed. I can see an unheated greenhouse might be helpful during the few frosty nights. However, the biggest problem we've had is keeping it cool enough--even in winter. So if you are making a greenhouse, make sure you have great ventilation with cross-breezes.
To find more on this type of greenhouse, look at the video I've posted above of Russ Finch in Alliance, NE. There are also many YouTube videos of walapinis and "underground" greenhouses. There is a guy on YOuTube who used to live in the northern East Bay and he had a giant garden--I think his site is called "Growing Your Greens"