Adding a Duplex to existing rental property Eire, CO

[Marcus Yoder]

Hi, we own a rental property in Erie, CO.  It has an existing rental home on it that generates positive cash flow.  The property is big enough that we could build a 2 story row-home style duplex on the land, and not have to touch the current rental home.  We would likely need to lower our rent, since we would be taking away a massive yard.

I am looking for an Excel worksheet template or some other calculator to help me figure out what the financial projections would be at the end state of having the current rental home, and then adding the duplex.  

Any suggestions?  Or any gotcha's I need to think of in executing this plan?