Do you think vacationers would view a SFH that does not have beach access or a SFH that is located a mile from a beach as a negative versus a condo that is right on the beach?

I access my clients' books via my QBO Accountant profile.

It's in the downtown location actually, there's a nearby creek about a block away. 

Does the rest of the neighborhood/block have attached buildings?

I tell my sales reps that if the seller is really tough to deal with on the first phone call, then we usually want to pass on it, since if they are that tough just to ask some basic questions, imagine what they would be like when we need access, are negotiating a contract, or God forbid need to try to re-negotiate terms, i.e. ask for an extension, or every once in awhile a price reduction if the repairs a lot more than we originally thought. 

Your attorney would have to access your risk profile to suggest your structure. 

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This is a great forum with very knowledgeable members that will help to guide you in the right direction.I really like the alerts feature that can quickly guide you to current topics that may be of interest: http://www.biggerpockets.com/alertsThere are a few self-contained guides that you can access here: https://www.biggerpockets.com/guides

We'll be renting the main house out when we move and the house tenants will NOT have access to the detached garage. 

They should also have all access to any permits, and will be able to help guide you on the rezoning for the ADU property.