Check your city/county, but in most places in CA and the rest of the US, zoning allows up to a 120 square foot shed without building permits. Neighborhood HOAs, if applicable, would be the other restriction.
36 square feet? good to go.
If I were to do it, I'd make it slightly bigger and put a electrical sub-panel in it with an extension cord like an RV has. You could plug it into anywhere with an RV outlet.
It's also easy enough to move any shed like this that's 8 feet wide or less on a car-hauler trailer or with dollys/rollers underneath.
Where are you seeing he used drywall screws? I use deck and construction screws for framing all the time, it's fine. You can always add nails too. 24" centers are ok for this too, there's very little weight. Far less well constructed sheds are moved daily.
Many sheds are built this way. Some even lighter - e.g. the overpriced wood sheds you buy pre-built use very minimal framing. And they all can be moved just fine. I commented because it's somewhat surprising how easy it is and how well it works.
I didn't say they were the same thing. And I meant what I said: zoning allows a shed without building permits in most places. It depends on the jurisdiction but because of model zoning codes the rules are almost all the same across CA.
In CA and most of the US there is usually no separate "zoning permit", although it exists in a few places (I see berkeley and sonoma county). Usually zoning and the development and land use codes just say what you're allowed to do, and are enforced based on helpful neighbor-driven complaints. But you usually don't need a permit up front except for things like conditional use and special use permits, certainly not a shed. For things requiring a building permit, the permitted use (zoning) is just part of that process.
The OP can and should check the city/county and find out. The point I'm trying to make is there is actually a fairly high likely hood that even in over-regulated CA you can build a 120 sq. ft. "shed" or "playhouse" in the backyard without a building or zoning permit. Without knowing the exact jurisdiction, this is just general advice about the most usual practices - for example San Diego, LA, San Bernardino cities and counties.
One thing I should mention: technically plumbing/electrical in the shed or calling it or using it as an "office" might not be kosher, but in practice everyone does this with no issue. It's just a very nice "playhouse".
36 square feet? good to go.
If I were to do it, I'd make it slightly bigger and put a electrical sub-panel in it with an extension cord like an RV has. You could plug it into anywhere with an RV outlet.
It's also easy enough to move any shed like this that's 8 feet wide or less on a car-hauler trailer or with dollys/rollers underneath.