While building a granny flat is less expensive than building a single-family residence, it is still an expensive investment. A recent survey conducted by the UC Berkeley Terner Center for Housing Innovation, ADU owners reported spending, on average, $156,000 to build their granny flat. If you’re converting an existing garage into an ADU, the cost can be significantly less expensive but even then you may still spend around $30,000 or more. Alot of urban areas within california (including San Diego) charge development fees. It is a good practice for homeowners to research the fee structure in their specific area to get an accurate calculation.