Here are some San Diego ADU resources and links to help get you started;  

San Diego City ADU and Junior ADU Guidelines & Links

ADU and Junior ADU Map of San Diego City, where ADU’s are or being built

Experienced ADU Builders in San Diego
Snap ADU Design Build 
Crest Backyard Homes
Jackson Design & Remodeling
Ritz ADU Design Build
 

Condo Conversions
San Diego City Condo Conversion Regulations
County of San Diego Condominium Conversion Applicant’s Guide
Adams Stirling Professional Law Corporation (manages conversions)