Build a garage loft?

[Jacob Rhodes]

I own a 3/2 in a desirable, middle-class neighborhood in Houston. I am considering adding a 500sq' garage loft (separate entrance, appliances, just shared back yard) and then moving out and renting both the main house and the loft separately. Very generally speaking, does the fact that another tennant would be sharing the property negatively impact the amount of rent  I can collect on the main 3/2 - if so how much? Is a separate loft generally considered worth the investment? I don't know of any other owners in my neighborhood that have done anything like this, and I'm wary of being the first. 

My other option is to use that space to do a cheaper (no kitchen/bath) addition that ties in w the house for an upstairs media room/office and adds 500sq' to my house size.

I know there are a lot of details I'm leaving out about my personal situation - but just a general answer is all I'm looking for from a more seasoned investor than myself. Thanks y'all! And thanks BP!

Jacob