detached rental unit on SFR1 lot ( ZA-120 ) Could use some help

[Justin Barnett]

I recently purchased a sfr in San Fernando Valley and it comes fully equipped with a non permitted studio unit built off of the garage. I plan on sectioning off back yard and renting this unit out. 

*Is there any way of getting this structure grandfathered in somehow in Los Angeles? It appears to be built in the 60's or 70's

*Also I have read thru AZ-120 and the subject property meets all requirements to have second unit with parking fully permittable. Is there a catch to this? It sounds to good to be true.lol

Assuming its not possible to grandfather in existing unit can anyone ballpark me on what it may cost to have the existing structure demolished, new plans drawn up and then rebuilt including permit fees, etc, etc. I will rebuild it at 315 sq ft and it will be the usual rental nothing fancy.

I am preparing for the worst but hoping for the best in this case and any feedback would be much appreciated, as to this is my first rodeo.

Thanks for reading

Justin