I currently rent out a couple rooms in my personal residence in Clairemont Mesa (San Diego, CA). I have a large backyard and want to add a stand alone unit to rent out. The total lot size is 5800 sq ft, the house is 1228 sq ft, and I could fit a clone of my house in the backyard if I didn't mind not having a yard, but since I live here too, I clearly don't want to do that. I have some contacts who can do the work, but I've never built something before so I was hoping I could get some advice on what to build that would allow me to generate $1300/mo. Some of my ideas are a simple one-story studio or one bedroom, or building one of those over a garage (to add value while minimizing footprint), but I don't know what square footage or other details I should be looking at to try to reach my goal cash flow amount (after the work is paid off). Any advice would be appreciated - as I mentioned, I haven't built anything before so I'm feeling pretty overwhelmed with all the options and trying to figure out ROI.

Thanks!