Estimating ARV for a duplex?

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Hey fellow BP members,

Our last deal was a duplex in Tallahassee with a 2br/1bath on top and a 1br/1bath below. We acquired it way below asking price and now we really want to maximize the potential of the place by adding sq/ft to the smaller unit downstairs by turning it into at least a 2br. My property manager has informed me that there are two ways to go about this:

1) We can build an addition into the backyard, which is relatively unused space. (This is the more expensive option)

OR 

2) We can build the extra bedroom into the existing garage space, which is currently being used only as a shared laundry area, and just make the laundry room smaller. (cheaper route)

I've been told before that estimating ARV for a duplex is harder than for a SFR simply because there might not be any "like" properties in the vicinity, which is the case here. My question is how can I estimate a fairly accurate ARV for both projects so I can compare which option is worth the investment?

I'll add that this would be our first time doing any big rehab so any and all feedback is welcome. Thanks!

-Kyle