New construction on Lot with Existing Duplex

[Shaun Hatton]

I bought a duplex in 2017 and turned a 12' x 12' shed into a 12' x 24' rental cabin.  I'm looking for any insight on how others with a similar situation handled it for tax purposes.  I want to be sure to get my starting basis right, recoup as much of my original investment as possible and depreciate appropriately.  

I'm assuming that the land will hold no value since it's already captured in the main house on the property.