Thanks guys, I appreciate the detailed responses!
Currently the place is a duplex, both 1/1's and my wife and I are living in one side which is the one I'm considering enclosing the patio on. Both have garages, 16 x 8 back porches just like yours Joe, and fenced backyards. As we are looking for prop #2 right now, I'll probably try to do the enclosure right before we move into our next property so I can immediately start paying off the cost of the addition with the increased rent. I can currently get around $900 in rent (that's what the other side rents for) and with the addition of the bedroom, easily $1200-1300, not to mention the added value to the property. That puts me at a little under 3 years to recoup the cost of a $10k addition at $1200 rent.
I'm thinking that going from a 1/1 to a 2/1 is much more valuable than going from a 2/1 to a 3/1, which is probably a more common occurrence.
What do you guys think?