Converting a detached 2 story garage into 2 more rentable units

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I am looking into a property with numbers that, at least on the surface seem decent (listed at $75,000 that last rented for $750, in pretty good condition over all).  However, what has drawn me to this property is the 3 car, 2 story detached garage.  I want to convert the garage into 2 more units to rent out to maximize cash flow.  So...

My Questions:

Anyone have experience with a project like this?

How do I go about checking the zoning for this, and what all should I be looking for?

What are the biggest potential problems I could run into when converting a garage into a rentable unit?

Any thing else I should know/be aware of?

More About the Garage:

It has separately metered electricity already but no plumbing or HVAC.  

This property is actually made up of 2 lots, side by side.  The garage is on the other lot, and has only a clear yard between it and the street (I assume that would make it easier to run water and sewage).

Only a one car garage door is left in the garage, the others have been framed in and covered.

The first floor is the size of a 3 (possibly 3 and a half) car garage.

The second floor is smaller because of the roof slant but is big enough for a large studio or a small one bedroom.