ADU - Does it Make Sense & How Much Should it Cost?

[John Cusick]

I own an about 3,000 s.f. house in a gated SFH community in coastal Southern California, and am considering building an ADU of about 900 s.f. Since I am doing it the first time, I'd appreciate some advice on the following issues:

1) How would building an ADU affect the value and marketability of the house? I am trying to avoid any unintended consequences . . . for example, may adding an ADU in a fairly upscale gated SFH community decrease the desirability of the house for prospective buyers when I am ready to sell? Any other negative consequences or pitfalls that I should consider?

2)   What should I budget for this project for the architect/engineer/permits? 

3)   What should I budget per SqFt for construction?

Thank you!