Looking to build a duplex in back yard in a rm1-2, rm1-3 zoning

[Tristram Coffin]

I am trying to  figure out what the law means for a rm1-2, rm1-3 zoning in San Diego. I have a 900 sq ft home with a 300 sqft detached garage. My lot is 5000 sqft and trying to figure out what would be the best bang for my buck. I have a yard that is 18 ft wide from fence to side of garage and almost 60 ft to the back side of my home. I do want to leave some yard for my home so not looking to use the whole back yard but the info online isn't helping much. I am looking for some help. Id like to build an adu in the back and in the future make the garage into a adu. My preference would be to knock down the garage and build a duplex. I am not sure if that is possible due to the wall of the garage is on my property line. If you guys could wither give me guidance or a recommendation on who to call, I would appreciate it!