Building anything in an incorporated city brings with it the obligation to not do anything dumb and thereby endanger your house or your neighbor’s. The constraints on sheds are pretty light, though. Here’s what I have learned.
Structures of less than 100 sqft, not on a permanent foundation, are allowed as ‘unpermitted’ structures. (This means ‘without a permit’ rather than ‘not permitted’. This kind of thing is only the beginning as you peruse city government web sites.)
I read that as long as a structure doesn’t have electricity and water, it’s not ‘habitable’ and thus on the good side of the law. This makes sense to me, as power for garden tools and such is necessary in a workshop. Running power to the shed, though, probably still wants a permit unless you do it with a ‘temporary’ extension cord.
I was told last night that a shed can be up to 8’ above grade, and that the 100 sqft is the ‘footprint’. Does this mean simply the part that contacts the ground? That seems unlikely; after all, many structures touch the ground only on a couple of 4” chine joinsts. It’s probably best to assume that’s the roof, but I doubt it’s a sticking point when it comes down to it. 8’ isn’t very tall, but given the light rain in California, I can probably get away with a 1’ rise over an 8’ width and have the lowest point be about 7’.
Decks are separate from structures, and you can actually deck or concrete a significant amount of your yard without involving the city.
IANAL; I’m just some guy. I’m in San Francisco; contents may vary in your jurisdiction, and I can’t be responsible for lost, late, or misdirected information.