Duplex have different zoning that may permit additional units.

Each contained historic use permits and have continually operated as 3- family buildings.  

@Ka Vangyes, you can use a HELOC for a down payment as long as your debt to income ratio permits getting a loan for the balance.however, i strongly, strongly recommend against it. 

The costs of land  , engineering , site work , permits , labor , and building materials make this virtually impossible . then there are zoning issues and the rest of the politics that go along with this . 

Let’s talk about your permit.

The only place they WILL NOT JV is in my home city of Philly... go figure. something about taking too long for permits and inspections.

The zoning will define the permitted uses (this will tell you if you can have an RV/Boat storage there) and it will define the density (this will tell you how many tiny homes you can add).You'll want to make sure you don't conflict the existing duplex to a new use or mixed use where you might put the existing use out of compliance. 

Nor is there a permit to do so. 

So yes timing turned out to be critical, as now those permits are like pinky rings.

Check your local (city) municode to see applicable zoning rules/permitted uses.