Considerations to make before building a room over garage

[Ankit A.]

Hello,

I have a 1963 built single story ranch style house, being used as my primary residence, in Dublin, CA. As a supplemental income I am thinking of building a studio over garage and renting it out as a long term rental or through Airbnb.

My questions are: 

1. Would it make more sense to build entire second floor vs building a studio over garage? 
2. Would it be cost effective to have separate utility connection (electricity & water bill) for the second floor?
3. Besides general contractors, is there any place online that would help me understand the entire process and potentially give me ball park estimate?
4. If someone has gone through this process, what are some of the gotchas I should consider?
5. What other considerations I should make?

Thanks in advance for your response.