We have operated STRs since 1999 and they had been around long before then (STEs have existed for many decades).  

There are less lenders that do this but they exist.

Not sure what the cost is, but I do know it exists.

The opportunity is what I love about this mortgage.

If it ain't broke don't fix it.You have a mortgage and insurance.

In my market zoning ordinances often make 95% of existing residential lots nonconforming where nothing could be built within the ordinance.

I actually have two options for this property: 1) Rent out the existing 25 storage units AND rent out the building plus add the 25 additional units out of pocket.OR 2) Rent out the existing 25 storage units and added the 25 additional units out of pocket to rent them out BUT keep the metal building for my GC business to where we have offices and storage for building materials.

I would then probably consider putting down the min. downpayment on a duplex up to a fourplex, for a place to live and rent out the remainder to  cover the mortgage and expenses.  

Given your strong credit, rental experience, and the equity in your existing properties, you’d likely have some solid options. 

It does seem competitively priced for the area.If anyone has insights into how the existing HELOC might impact the mortgage assumption or tips for negotiating on a deal like this, I’d greatly appreciate it.