Too many laws in too many places. [I know I won't consider the UK now :->]
EDIT: I have heard about this zoning concept, it's just a bad idea IMHO, especially when applied to the modern tech workers. In France, unless you run specific equipment (say heavy machinery) which can create nuisance or risk for the neighbours, or if receive the public (wear and tear being a nuisance in itself), you're quite free. Zoning as in simcity (residential/industry/commercial) is a dated concept. I want to leave near the place I work and where I can find commerces.
In France, there are even special corporate statuses for single entrepreneurs, which especially allow you to do that in case your landlord may tick, and even deduct part of your home expanses as business expanses (say if you are using a room of your 4 bedroom flat, 15% would be reasonable because you'll still use the bathroom and the kitchen more for yourself that for your company, the % amount is left to the appreciation of the tax agents)
That's one of the rare business friendly laws, even if I must say I've been quite surprised by the recent Hollande plan, which for the first time seems to talk sense about business and employment.
6 people in an apartment sized for 4 may be a nuisance.
"Commercial" activity generally refers to customer traffic, not personal work.
As long as you don't need to have frequent visitors, aren't making tons of noise, and generally make it look like your apartment isn't a makeshift office, I'd probably be more worried about my landlord finding out than the zoning commission.
Is commercial office space substantially more expensive than residential? I could see using a space you're living in as an office, but if the original tenant moved out because the office was infringing on his space/life, is it cheaper to maintain the residential apartment as an office or find proper office space?
http://romero.smugmug.com/Video-Games/The-Archives/480_GtbLP...
The rag-tag office setup is pretty inspiring.
2 bd w/ living room apartments were cheaper than office space across the street.
Caveat: I'm in the UK. Things may be different the other side of the pond (and may vary by state).