ADU cost - for estimates

[Pandu Chimata]

Hello team, How is everyone doing? I recently bought an house in San Bernardino City and doing the rehab. 

This is a small house with 850 Sq footage ( 3 bed and 1 bath) and also a smaller lot of 5100 Sq Footage. 

While doing a rehab, exploring an option of adding an ADU as well. I was told by city that the ADU has to be between 800 and 1000 Sq footage.

Now I am evaluating whether it makes sense to add an ADU or not.

Per my relator, it does not worth from listing aspect, as he does not see lot of data that supports the return for the ADU investment? Thoughts?

FYI - Its going to be an ADU from the ground-up since the existing garage foundation may not work for the ADU.