The rules UCGal posted match up with my understanding as well.
When I built my detatched garage in Clairemont, the inspector was very leery of anything that suggested I might be planning on sneaking a kitchen or bathroom into the garage. So, the city pays close attention to this. I don’t think it is something you could possibly sneak by a planner.
I think that Agricultural-Residential lots allow the second structure even if the the lot size is not 2x the min. I’m not 100% sure about that, though. Still, I would expect the more rural environment of Poway to be less restrictive on this.
