That makes me wonder what else you could repurpose as an office in places where offices are disproportionally expensive. What about a climate-controlled storage unit? A quick Googling tells me you can get a small (but large enough for a desk) climate controlled storage unit in San Fran for about $100-150/month. That's vastly cheaper than a co-working space. I know you can get storage units with power, they're pretty popular as practice spaces for bands around where I live. You'd need a mobile hotspot for internet, and no windows might be a bit depressing, but for the price...
Met a guy in DC that actually lived inside a warehouse storage unit. He had installed lights, walls for multiple bedrooms, a kitchen/bar, and a lot of cool art. I'm awful at estimating distances but I'd guess the main room was about 800sqft and the bedrooms/bathrooms were divided from another 400sqft. With a little love, any place can become a home.
Every warehouse/storage unit I've ever leased has included terms that specifically prohibited doing that, though. You might be able to get away with it for a while, but you should always be prepared to be evicted.
I think he meant rented warehouse space, not an actual storage unit. Storage units rarely have more than a single electrical outlet and many lower end ones don't even have that.
Most of the plumbing technology developed for RV 'boondocking' would be perfectly adequate. Bit of a PITA, but I've done that for 2.5 week stretches in the mountains on ~1 gallon/day of H2O. My real concern would be no windows, definitely not great for anyone who suffers from seasonal affective disorder.
