So many different "opinions" are likely to pop up here, but here's my 2 cents...
(remember the Realtors are Realtors, not appraisers)
If you are using it for a garage, NO.
It would have to be an enclosed building like a casita or guest quarters. No garage door.
To make this simple, since it is separated from the home, it has to be almost it's own home, not a garage style building.
So, if it looks like a Duck, Walks Like a Duck.... You get the picture.
Does that make sense?
Hope this helps you. I have 2 appraisers I work with that I can provide you contact information for that you can speak to regarding this if you are interested. They are local and highly experienced and would probably be the best resource for you to conform this information.
I'm always here to help.
Ph (480) 220-7491
Fax (866) 711 8573
Keller Williams - Scottsdale, AZ
2008-2009 Master of Real Estate award recipient
ABR, GRI, ePro, CNE, MRE, AHWD